There is a lot of discussion of this in the archives, of course. In particular, there are a couple of good threads about 500 series units that you would probably be interested in.
My standard recommendation is the API 3124+. Four great channels at a good price. Sounds excellent on just about everything. I really need to get another one so I have eight channels of 312 for drums. I'd start with that as a "foundation" preamp and add others from there.