If you are set on gettin an external compressor here is what Henry (D.A.V.E) said when I asked him this same question a year ago (before I descided to get the UAD)...

"the TLA ivory series valve compressor (5021?)is great for warming things up at a good price. really good for a whole mix or on a sub group for bass and drums together. if you're after something tighter for drums and compression effects maybe try the drawmer dl 221 (or its modern equivalent) which is a bit pricey but will really get those kiks punching. also there's a budget focusrite around it's one of the platinum series and is a basic stereo compressor with presets for drums, vocals etc but you can also set it up manually their stuff is always top quality and definitely worth a try focusrite also make a blue range (eq's ,comp's) expensive stuff but definitely the best there's a lot of stuff around for these jobs but a lot of the budget stuff really isn't worth it. Definitely stay away from behringer and alike."

peace
-G