Hi there " hobokenny ", and Welcome!
Wow! You sure do have quite a dilemma on your hands, with no simple solution. As Greg said, if that building works for you, and you can make a good business case for buying it, then that would be a reasonable option. But if break-even is five years down the line, and the building isn't isolating very well to start with, then it might be better to bight the bullet and start from scratch. However, in that scenario, it's going to be at least a year, probably more like two, before you get any income at all from the new place, and then you have to re-build your goodwill and your standing and entice all your current customers back again.
I see the source of your quandary and indecision. That's a hard call to make.
I can't help you make that decision, but I CAN answer your specific questions:
A) what should I be looking for in a building? An empty shell? An office to retrofit? I’m assuming single floor. How about height?
Open space on the ground floor (not an upper floor), concrete "slab-on-grade" floor (no basement below: slab sits directly on the ground), brick or concrete outer walls (preferably), concrete slab ceiling above your head (preferably), ceiling as high as possible but at least ten feet, existing HVAC system already dimensioned to handle the worst-case load of having that space fully occupied by dozens of people and a lot of musical instruments, gear, lights, equipment, etc., plenty of parking directly outside for customers, easy access from major roads but not right next to a freeway, not on the approach or departure path from an airport, not close to a railway line or station. Alternatively, an empty lot that fits all those location requirements, then build your place from the ground up, as a purpose-designed rehearsal and recording studio.
B) Construction labor rates are quite high at this time thanks to the booming economy, and the last thing i want is to end up with double costs in twice the timeframe. Are there any tips here on managing this? Are there pre-fab materials I can use to get a better handle on exact costing? Or would that be astronomical in comparison? In a perfect world I would want to be able to accurately cost the 180 sf practice pods, and then get creative with the larger spaces to accommodate to the crowd that would be willing to pay more daily/monthly.
The best way I know of to estimate the cost of building a studio from the ground up in any specific city, is to check around with local building contractors and ask for their flat rate per square foot for building an upscale home of a few thousand square feet in the same area. Then add 25% to that. That's a realistic estimate, but it still wont be accurate.
For refitting an existing space, call those same contractors and ask for their flat rate per square foot for finishing an bare-bones unfinished basement or garage, turning it into a home theater or "granny apartment". Add 25%.
Are there pre-fab materials I can use to get a better handle on exact costing?
No, because nobody makes pre-fabricated sound isolation walls! Every studio is different, and each one needs to have its walls, floor, ceiling, doors, windows, and HVAC system designed for the specific requirements of that studio, that owner, and that city. There are only ball-park estimates, such as above.
In a perfect world I would want to be able to accurately cost the 180 sf practice pods,
As a very, very rough estimate, my clients who have built studios report that, for most of the USA, it costs about US$ 100 - 200 per square foot to build a studio from the ground up, not including decoration and equipment, with the average being around 160/ft2. And for renovating an existing place for the same purpose, that is in the range US$ 50 - 120. So there is an overlap: in some cases it can be more expensive to renovate than to build from a blank piece of land (usually when the existing place is old, or in poor shape, or not well built, has no HVAC, etc.)
I know those are large ranges, but they are realistic, and come from real customers who have build real studios over the last few years. Of course, "your mileage may vary", "caveat emptor", etc....
Costs vary by city, and location within city. There's lots to take into account. But those are realistic.
C) are there specific “gurus” in this industry that cater to these types of buildings? In the past couple weeks I have gotten a lot of head scratching from financiers, contractors, and architects who can’t quite wrap their heads around the concept of these spaces. It would be great to find a resource to help me get right to the point.
Yes, there are people like that, but we aren't so easy to find: we hide in places like this forum...

What you need is a studio designer to design the place for you, THEN you need a local architect to walk the design through all the local red-tape, THEN you need a contractor to build it. Do not, ever, under any circumstances start with the contractor! Do not, ever, under any circumstances, allow an architect to design an acoustic space! Do not, ever, under any circumstances, allow your studio designer to also be the architect or contractor! Let each one do what he does best, within his own specialization. Architects get about 5 minutes of training on studio acoustics during their entire university courses (OK, maybe a little more, but still next to nothing). Contractors might be able to build a fantastic structure, on-time and on budget, but they know nothing at all about designing studios. We studio designers know a bit about construction, and a bit about architecture, but not enough that you'd want to trust us to actually build you anything more complicated than a dog kennel! Get the right team together, get them working together, but let each guy do what he does best within that team, and try to minimize overlap. Don't let your architect change the specified acoustic materials without checking with the designer. Don't let the contractor change the construction techniques to save you a few dollars, without checking with the architect and designer. Don't let the designer specify exotic / expensive materials that the contractor can't even find in your location, and that the architect doesn't have a clue how to incorporate. Etc.
D) Am I completely crazy? Any sage words of advice from someone who’s ever been in a similar situation?
Well, you are building a rehearsal / recording studio, so yes you are crazy! If you were less crazy, you would just build an office block, or a supermarket....

But you are "good - crazy" not "totally mangled mind - crazy", so that's fine.
Oh, and by the way: there's wasted space and unnecessary complexity in the layout you show on your existing plans...

I'd guess that it was designed by an architect with some background in studios, but not by an experienced studio designer...

To start with, it's actually a myth that your walls need to be non-parallel. A very well regurgitated and never-endingly circulated myth that refuses to die, but a myth nevertheless. The only valid reason to splay your walls is to get rid of something called "flutter echo", but to do that the walls need to be splayed by at least 12°, which yours aren't, and there are far simpler ways of controlling flutter echo. Splaying walls just complicates construction, making it harder and more expensive to build. Acousticians understand this. Architects don't.... It looks cool and sophisticated too, but is not necessary, acoustically. So, that's wasted space, wasted effort, and wasted money. A seasoned studio designer would probably not have done that, unless you specifically insisted that you wanted splayed walls, or unless he needed to do that to fit the rooms in better.
Bass still travels in the building a fair amount due to the floor,
Yup. Not isolated properly. Especially considering the large amounts of materials used, and the large air gaps between walls! There are clearly unresolved flanking paths in there, probably thorough the floor and also through the HVAC system. Those are the usual culprits. Also possibly if the inner-leaf ceilings of the rooms are not correctly isolated, and if the inner-leaf walls are mechanically connected to the outer-leaf at any point except the floor. Beginner mistakes....
Ceilings are suspended individually in each room and use a glue adhesive against the walls of each room to help reduce vibration.
Wrong! There's your problem! That implies that the room walls are attached to the building structure at the top, with the ceilings hung inside. So the walls are flanking. The correct way is to have the inner-leaf ceiling resting ONLY on the inner leaf walls, and have no part of the inner leaf in contact with any part of the building itself, except the floor. That's why you have isolation issues, despite the large mass and air gaps.
Floors are concrete; no enhancement or additional padding there.
Fine, but for that situation (20 rehearsal rooms together), I would put decoupled drum risers in all the rooms, to reduce impact noise getting into the slab. I would also specify laminated flooring on acoustic underlay. Thick, tough, high-traffic laminated flooring, such as is used in shops and high-traffic areas of office buildings. Not the thin flimsy stuff used in houses. And proper acoustic underlay, not typical thin underlay.
That plan does not truly isolate the floor, of course (way too expensive to float the floors properly for 20 rooms!

), but it does help to reduce impact noise and keep some of the air-borne noise out of the slab. Studio designers know this: architects and contractors don't...
We have had quotes all over the map as to what it would take to re-create those walls/ceilings in an empty box...from $25-50 / sf for materials alone to $250 / sf for a full staffed buildout.
The materials quote is on the low side, and the full build quote is on the high side. Realistically, it would be more likely around 150 - 200 /ft2. HVAC is the big unknown that can hit your pocket very badly, if the building doesn't already have a suitable system.
To add to this, I am not handy with anything that's not a board, rack, stack, or an XLR snake.
Hire a contractor. A job of this magnitude is far too big for a single person to handle. A typical home studio builder can do a studio of maybe two or three rooms inside a couple of years, but 20 rooms plus service facilities plus office facilities is major. Hire a contractor, and hopefully one that has studio experience. Building a studio is NOT like building a typical home, shop, office, church, school, etc. Major differences.
This forum gives me a ton of inspiration that, perhaps, I COULD do a lot of this myself, but I'm unproven at the moment...
You MIGHT be able to reduce some costs slightly if you talk nicely to the contractor and he allows you to do some of the work... but more likely he would charge you MORE if you want to help, as you'd be in the way, and his people would be tripping all over you, bumping in to you, and fixing stuff you did wrong...
If this were going to be your own home studio, I'd certainly say "Go for it! Do it yourself!", but a 20 room commercial facility is beyond the means of a single person to build in any reasonable amount of time. It took Noah and his sons over a hundred years to build the ark, reportedly....
As an alternative, if we find the 'right' building, and phase in construction the 'right' way, we could put those hundreds of thousands of dollars to better use...somehow.
Think this through, ball-park: You want 20 rooms of 180 ft2 each. That's 3,600 ft2. You will be required by local regulations to have a certain number of bathrooms, some of which must be ADA compliant. You will also need office space, waiting rooms, storage space, maybe a kitchenette, so allow at least another 800 ft2. I would also suggest that you put in a simple control room/mixing room, so you can offer the additional service of recording the sessions of your customers, and doing a rough mix, so they can go home with their own demo album. That room is "premium", billed at a much higher rate, of course, so it's an extra income stream. That's an extra 300 ft2. Plus passages, etc. Total floor area: around 4,500 ft2 to 5,000 ft2. Going with my ballpark figures above, this would cost you around US$ 450k to 1m to build from the ground up. Likely around US$ 600k - 700k, realistically. Plus the cost of the empty lot to build it on. Alternatively, if you can find an empty warehouse, shop, etc. with about 5,000 ft of floor area and mostly meeting the criteria I outlined above, then you are looking at something more like $250k to $600k to renovate that into your rehearsal studio, and likely around US$ 400k. Plus the cost of buying the building.
Are those options cheaper or more expensive than just buying the existing space? That's the basic question. If it is a LOT cheaper, then go this path. If buying the building is cheaper, even with a 40% premium on top, then buy the building!
But do take into account the down-time. If you buy the building, you can start renting out the studios tomorrow, so you have immediate income. If you build, then it will be a year to 18 months before you get any income stream, and your existing customers will have walked, so you'll need to seduce them back again, with special offers, free sessions, perks, etc.}
It's a really tough call, for sure, but you are the guy with all the numbers. We can't make the decision for you! But hopefully I've given you enough additional and realistic information to help you make that decision.
- Stuart -