I'm looking for a reputable shop, perfer nondealer to look over a s4 i'm considering to purchase. where do most people go or recommend

Drive up to Madison and go to Kelly moss motorsports
http://www.kellymoss.com/
Did a pretty good job on my A4, as far as milwaukee I've tried autobahn and autox in milwaukee, both werent that great (had some bad prob with both actually) and I wouldnt recommend them to anyone.

Best place in Milwaukee (actually they have a few locations) is Engine & Transmission Exchange...they did a great job with my A6.