Search! theres many many threads about this.

You may be able to get one for under $10,000 but its still a $40,000 car to maintain.

T-belt should be done sooner than later. Some people have oil leaking problems. The turbos are unrelyable at anything above stock boost, and even then poor oil line location makes them prone to failure. If you want a manual transmission know that your 01e transmission will probably need to be rebuilt someday because the tolerences on some parts are too tight and will bind. Theres a long list of cheaper (under $1000) common problems like the throttle body boot tearing, tie rod ends, dead pixles in the display, some bad sensors, poor diverter valves.

Furthermore as a car gets old its going to need work so expect it. Parts are expensive, but labor is too and some jobs require many hours where they wouldnt on another car with more room under the hood. Working on your car yourself will save you ALOT of money with a B5 S4.

Its a great car to drive but if you made a list of relyable & easy to maintain audis of the past 10 years this would be pretty near the bottom IMO. Buy one but make sure you know what your getting your self into. I got one with low mileage (53,000) that had been taken good care of and in 3 years iv still spent more than a few thousand dollars without really upgrading much.

Why would you put up with this? Because the B5 S4 is one of the greatest sleepers of all time. With just a few modifications a Stage 2 car makes well over 300 HP at the crank, and plenty of torque comes early in the powerband. Leather seats and 4 doors make it practical, Quattro AWD makes it usefull year round not matter where you live. On the highway you can get respectable MPG. Plus NOGARO BLUE!!!

You can easily get the mod bug with this car because the "standard" turbo upgrade can get you 400 HP at the wheels with proper supporting mods. Then theres GT turbos....