I work on them for a living - and do your homework with this car. Unless you have the money to spend on these things they get VERY expensive VERY fast. Here are some issues I've found..
-Instrument cluster center display will dim/not work
-Blown speakers (Bose replacements are $$$)/Blown amps (again, $$$)
-T-belt replacement/waterpump/thermostat - T-belt on these engines is a bitch, and if you screw it up, you instantly will bend 30 valves. Thermostat and water pump must be done with the T-belt.
-Secondary coolant pump/valley leaks - big time leaks under the IM on higher mileage cars, secondary coolant pump can fail or leak as well. Not a cheap item.
-Boost leaks. (Have you seen under the hood?)
-Ball joints an tie rods tend to wear on higher mileage cars, and the S4 had 2 upper and 2 lower balljoints, and the control arm bushings wear out too.
-Brakes get expensive, unless you tackle them yourself, and use aftermarket parts. OE rotors and a set of pads is close to $800 for the front.
-Leaky camshaft/adjuster seals - replacement means a partial teardown of the engine, removals of cams, etc.
-Torn CV boots/leaky prop seals/axle seals.
So again, buyer beware. When it runs normal, it's a fast car and a lot of potential, but lose track of preventative maintenance and it gets crazy in a hurry.
