get a nice compressor for hot sounding vox. Some people will say that aint the way to go, but you aren't trying to please 50 year old sound engineers, your trying to please the listener. If your lookin for a hot sound, like the lox use, try waves c4 if you work on a comp. It really brings vocal presence------the only other advice my unknowladgeable ass can bring you is to have a rapper with a nice delivery, because thats what really matters.
As far as the booth, I'm in the process of designing one myself-----you start using these condensor mics, and they pick up everything(and I mean EVERYTHING!). What I have in mind, is basically a 6 lvel wall all around. Probably be carpet on the outside, then plywood, a 2x4 frame, next will be a 2 inch airpocket all the way around, dense insulation, another level of plywood and then more carpet on the inside, a 2x2 window, and make it 4feet by 4feet and about 7feet tall. This is my plan anyway, hopefully it will work. most places have an eight foot ceiling, and I don't want it to touch anything except the floor, as far as the preamp, try them out before you buy them.