diffusors can help you get a bigger room sound. they help get rid of early reflections(compared to a flat wall) and create a somewhat more diffuse space.
you shouldnt be too concerned to have everybody go di. you can get drums and guitar in the same room with a few gobos around the amp, but still enough level for people to hear, and there can be very little bleed into the drum mics.
my main concern with a room like that would be getting good mic placement. you cant really monitor your drums mics as you put them up to hear what they sound like. i would check out some of the isolation headphones available. the most attractive ones ive been able to find so far are these ones with 7506 drivers in them. check out this thread.
http://gearslutz.com/board/showthrea...5&pagenumber=4