Just curious, is isolation a goal of any importance (do you live close or far to neighbors?).
Staggered studs with plenty of fiberglass will help isolate, and will help with your acoustics as well. Start pricing large bulk amounts from local sources.
The walls do act as reflectors, but that's not a good thing, at least for 99.9% of speakers made, especially with any close distance to either speakers and/or listeners. You treat these reflections with broadband absorption.
HT rooms are desired to be very dead.
Some folks deaden the entire front wall and ceiling as well. It all depends on how nuts you want to go, and what kind of budget you've got.
Diffusers need space to work effectively. I've never considered it to great extent; all I use are large broadband absorbers. I did once briefly consider acoustical cotton for a strange application.
Give a call or email to some acoustical companies like GIK or Real Traps. They should provide you with some good guidance. A couple of GIK folks chime in here time to time, and hopefully they will see this thread soon, and perhaps by that time you can post photos for them.
For inspiration, here's the only garage thread I know of.
http://www.avsforum.com/avs-vb/showthread.php?t=574704&page=2
http://www.smxscreen.com/images/IMG_5261-1.jpg