Sticker on my 2000 S4 was $41,400 loaded with everything except the navigation system.
Here's a breakdown of my service records:
Oil changes and tire rotations/balancing--for some reason, the tires need to be re-balanced every 5000 miles, go figure.
This car chews up tires, I've got 40,000 miles on the car and have eaten through the original RE040's (only lasted 12,000 miles), are nearing needing replacement of my current set, and have pretty much chewed up my set of winter tires (on seperate rims). Perhaps it's just the way I drive.............
Clutch was weak from day one and started slipping and needed replacement at 35,000 miles. According to the service tech I probably had a bad clutch from the get go and the new clutch is much stronger than the original clutch ever was.
Cheap mods: Chip and exaust (both from APR) were less than $1500 combined. Can't remember what I paid for my UUC short shifter (40%), but it was worth it--the stock shifter has unbearably long throws IMHO.
Car is fast. Mine's black and is definately a stealthy sleeper. Lots of 'Stang GT kiddies don't have a clue what it was that just blew their doors off.
This is not Audi's first turbo. I believe it's the same turbo (just two of 'em) that VW/Audi have been using in their 1.8t's for many years now. Audi also had Turbo's in their previous generation UrS4.
If you find an S4 in good shape and do buy it, you won't be disappointed. If I were to buy one, I would buy one that hasn't been chipped: The car is "undertuned" at the factory, that and the fact that is has a rev limiter will ensure against damage caused by hard driving.
Good luck with whatever you choose.