As the very satisfied owner of a 2007 S4 Cabriolet 6MT with 90K miles, I applaud your taste in motors, sir.
Two things I did before purchasing my relatively high-miles-for-model-year B7:
1) Thorough PPI by a reputable Audi specialist, including a compression test and boroscope. The cylinder wall in B6 and B7 4.2 are prone to scoring if pushed too hard immediately after cold starts. Easy warm ups from cold start to operating temp (90°C) are recommended before pushing rev counter past, say 3000 RPM.
2) Make sure you are able to get PPI with a cold start. The problems cited with with the B6/7 4.2s are related to timing chain guide wear/failure that is most detectable from a cold sump start. Once the engine is warm, the sound of the chain rattling on worn guides, is far less noticeable.
I was very concerned about potential service costs on the B6/7 S4, but being comfortable with the chassis/platform, I felt much more confident once I saw that the previous owner(s) had maintained good service habits and the internals of the 4.2 were in very good working order.
Good luck with your purchase!