only allroads had timing chains
I have a 2001 4.2 and my brother has a 2002 4.2 sport and they both have been very reliable.
I drive the piss out of mine, 110+ on nearly a daily basis and love that quattro cornering =]
If you're looking at any audi (except allroad) make sure you know the service records regarding the timing belt. If you don't, get prepared to dump the 600+ bux for a do it yourself kit or get raped at a mechanic for the service, $2500 or more. The job really isn't that hard, these cars were engineered with service in mind. I just did mine at 71k and it was very ready to be replaced. Audi claims a much higher service interval for timing belts but I wouldn't go a mile above 70k on one. Over all it took about 6 hours and I was constantly amazed how well engineered car is, it's a good way to get to know your car too.
Of course, with the kit you get a water pump, accessory belt, cam seals etc.
bhb399mm is very right, I have friends with S4's and 2.7t's. They always seemed to be worried about something or ripping them apart. When I bought this car I wanted something that I could beat the holy crap out of and not worry about it. This is, by far, the best car I have ever owned. It has taken way more punishment that my 540i or the M3 before that.
Just maintain it and don't buy a car that wasn't maintained, treat it right and it'll treat you right.
FYI - The chipped 2.7t's will have you up to about 60mph, but thats where the 4.2's really start getting angry and moving like freight trains. A very noticeable difference when you drive either one.