Due to the different wheel types games use, and that the good mid-90's and up racing games run slow as crap, I don't think I'd enjoy a MAME based driving cab.
I'd either use a PS-2 and Grand Turismo on there, or just use a PC for PC based driving games. PC racing games like Need For Speed: Underground and the Colin McRae Rally series are way better than anything in MAME.
Not trying to poop on your buddies idea, just make sure he understands what he's getting so you don't do a lot of work for nothing.