OK, first off those are all really bad room ratios and will make everything you record/listen to sound like crap -
Second, if you want this very sound "proof", you'll need to allow at least 1-1/2 to 2 feet more in EACH dimension of the building for soundproofing;
Third, you'll need to look at (and either get brochures on, or take pix of) the "candidates" you're interested in - specifically the floor frame, walls, door frame, etc - for this, you do NOT want an insulated building; that will only get in the way of soundproofing. You want a bare frame with metal skin, period.
Fourth, you will need to find something with taller ceilings if possible, and a VERY sturdy floor; either that, or you'll need to find something that the bottom floor frame can be reinforced - otherwise you'll find yourself and your gear breaking through the floor if you've done much toward getting good isolation.
I know there are people (including yourself) who are curious about what I have in mind; the simple answer is NOTHING (yet) - let's see what you find for "candidates" first. Besides, who doesn't like a good mystery? Speaking of which, what's your approximate budget before we get too far?
Hey, instead of doing this, why not get a good-sized "hi-cube" van, with the big engine (to pull all the extra weight) - then you can do "location" work as well. You're only using 8 x 12 spaces already, just make 'em end-to-end instead of side by side... Steve
