As you can see in the picture . . .
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the frame the door will sit in will be quite small
Door frames cannot be small! They need to be massive. Normally there are at least three studs on each side (two kings and a jack) as well as a header with cripples, plus multiple noggins. That's the minimum. Like this:
GENERIC-door-framing-detail.jpg
AGGLAS--UK-V6-S0143-door-framing-detail-2.png
door-framing-three-studs-GOOD!!!.jpg
BRAUS-door-framing-detail.jpg
I will use extra fixings on this wall as the door will add some movment if not tied in well.
I'm not sure I understand what you mean by that: How would it even be possible to build a door that was not firmly fixed into the wall?
I will be getting a complete door with no holes so I can decide on the locking mech i will use.
Your door should be solid core, not hollow, and you should drill any holes at all through it. Use only surface mounted handles, and an automatic door closer.
For the ventilation I am looking at various HRV units with summer bypass to provide air I am not yet considering Aircon. In the UK its only hot for a few months
Heat is not the issue. Humidity is. You live in the UK where humidity is pretty much always high outdoors, and you need to keep it at a constant 40% RH inside your studio, for your instruments, mics and gear..... An HRV unit does nothing at all to control humidity. Only a proper air conditioner can do that. Many people use mini-split systems for that in studios.
- Stuart -