pole barn wall question

Hello, I posted the following on the construction forum and was advised I would do well to post it here in the design forum as well. I'm in the process of selling my house in town. So I'm having to abandon my existing studio. It was 8x13x18. Not a bad studio, but I had constructed without doing any research and mostly going off of the "myth's" that are out there when constructing. Hence, no loud recording at night and many trips back and forth to my truck when mixing down. I just purchased a piece of property out of town that has a 30x50 pole barn. 4x4 wood post frame. Metal sides. The distance from the floor (dirt right now) to the bottom of the wood truss overhead is about 12'. The barn is completely empty. I would like to construct inside the barn, but before I did anything I decided I would try to educate myself a litte more about what I was doing. I have bought and read Rod's book, read quite a bit of info on the John Sayers site and the SAE site and just recently this site. So, although I still have many questions, I feel I have a good idea about the way things are "supposed" to be done. I would like to keep half of the building "open" for storage and other things, so that leaves me with a 25x30 space. So far I have sketched out a control room, vocal booth, main studio, storage, bathroom, small shop in this space. Also, a "tv/recreation" room. Anyway I have all this in a 25x30 space, and I'm somewhat happy with the layout. I'll probably be going with 8' ceilings to give room for a/c ducting and storage above ceiling. Anyway, on to my first question. I would like to utilize the walls of the existing barn somehow and then build another wall inside of that. So far I have thought of: 1) Cutting drywall to fit in between each wall post and then fill with insulation, but: 2) I live in southwest Louisiana (extremely humid) so I was thinking moisture may effect the sheetrock that would be against the metal so maybe I should put some sort of vapor barrier (tyvex) before putting the sheetrock in or: 3) Instead of sheetrock use wood or: 4) Don't use drywall or wood and just insulate in between the studs. I've been pondering the best way to utilize this existing wall for some time. I tried to add up the cost for lumber, wiring, insulation (a guess), drywall, lights etc. Everything I could think of (I'm sure I missed some). I came up to about $6000 not counting the slab I'll have to poor and also not counting A/C. I can live with this amount and a little more. Any suggestions on this subject would be greatly appreciated. Oh, I record anything from just spoken voice to full all out R n'R band. The nearest neighbor (besides myself) is about 75 to 100 feet away. That's just a guess though, I haven't measured. It may be a little more. The next closest neighbor after that is about 100 yards and they are family and will probably be helping with the build. So if they hear anything at their house I can always tell them "I told you we should have _______" Thanks in advance for any advise.