If you are responsible for regular maintenance/ insurance I would stay away from an S4. If you want an Audi I can get you some links to some 1.8t's that are in good condition, or even some S4's if that would interest you.
As for millage on the engine, as long as it is taken care of, the engine will last you a long time, I've seen
4Runners with well over 200k and on an Audi the best I've personally seen is 300+K, Saabs can get well over 1million apparantly.
So then what are you looking for in a vehicle? Speed, style, getting anywhere you want to go? with your budget there are a lot of options if you know where to look.