Don't ever buy a used turbo unless itis from a good friend. I've seen so many people buy used turbos that are "mint" and they are fucked from the get go.
There are some good affordable a/c compatible turbo kits. Peakboost, AFI, and if you got the cash full-race. There are some others like the Treadstone kit that are real cheap and have decent components but not top of the line. LHT's stuff is real nice and the install is top notch, I don't know how much they charge these days so my advice wold be if you can install your own kit with confidence then buy a complete kit like AFI, if you can't do the install contact LHT about their kit. Both John Vega and LHT John are very competent tuners so you will be in good hands either way.