Covered the structural issues (which are no true issues at all, just a matter of the right materials), let's check the proper treatment inside the room:
I just HATE going to a rehearsal room and find it covered by absorptive foam which deadens everything except the bass and the kickdrum, making it a low end echo chamber. AVOID THAT at all cost.
First and most important thing: Bass traps. I even build mines myself. But since you'll have taiko drums, you'll need plenty.
Ideally, you'll need at least two to three basstraps for each corner (floor to wall corner, wall corner and wall to ceiling corner). Basstraps will make a day-to-night difference in the overall acoustics of the room.
Good thing is, basstraps can also absorb high frequency energy, so everything will be balanced without spending hundreds and thousand of dollars in foam.
Basic basstraps pannels for dummies: 120 cm tall, 60 cm wide and 10 to 20 cm deep rockwool or fiberglass, covered in some soft fabric. You can even do a nice wood frame for it. And remember not to lay it ON the wall nor the corner, but a good 20 cm of the corner for better absorption.
You can build your own traps for.... $10 or $15 each.
Now, basstraps are covered, and if laid on the corners, standing waves should be covered too.
Instead of adding absorptive material for high end, try using DIFFUSERS. Plywood is a really good and cheap diffuser material. There are several designs for more complicated diffuser, but the easiest and cheapest ones are CURVED PLYWOOD PANELS. You can lay several of them in different acoustic sweetspots across the room and having an uniform response everywhere.
Now, some extra money-saving tips:
* Soundwaves don't bounce on paralel walls pretty much... never. If you plan on treating one wall with absorption, the wall right in front of it doesn't need it so much. Same goes with ceiling vs. floor. If you plan on laying carpet on the floor, the ceiling can be untreated.
* Diffusion works like a charm, and makes it feel like you're swimming in sound. Try using absorption only to correct acoustic disbalances and use small diffusors in different points of the room.
* Basstraps are a must. High spectrum basstraps like the one I said earlier work better than foam for every frequency band.
Ok, I'm off to lunch. I'll be back later. Feel free to email me at firstname.lastname@example.org
for further inquiries. I check my emails 10 times a day, so it's no big deal