I had an S4 for 18 months. It was approaching 100k (sold it on 99k or something), and I never had any real issues with it. But it is supposedly an problem if they are too rattly. Mine had a very slight rattle, but nothing too bad.
I believe it's an expensive, engine out job, if it needs doing though. So be prepared to walk away from any that rattle too severely.
Clutches are due to go around that mileage too though, so be aware of them. The autos are meant to be more economical on the motorways, as they sit at lower revs at 80 mph and such, but you don't buy a car like this for economy (although mine wasn't bad, averaging 23 mpg over my time with it, around 10k. Expect a couple more mpg out the auto if you do a lot of motorway, a couple less if you do a lot of a/b roads).
Better of checking out more S related forums for more in-depth infor though. RS246 sad SRS forums are both pretty good, and will tell you what to look for, over and above normal B6 / B7 issues.
