Like any VAG car of the era, only buy one that has documented maintenance at a VW-Audi speciast. The B6 1.8T's maintained at random garages are usually POS's, they will have the wrong oil filter, they rarely put in the correct VW502/505 oil, so you can expect sludge in the engine, and replacement parts brakes and suspension parts are usually cheap aftermarket stuff instead of OEM quality, meaning you will likely run into expenses sooner than you'd want.
That said, the common issues or areas that require frequent repairs on these cars, even the well maintained ones are:
- MFD display malfunction
- Oil sludge (if incorrect oil is used)
- Interior trim flaking away
- Valve cover gasket
- Ignition coils
- Control arms
- Wheel bearings
- Sensors (o2, crank, oil)
- rear coolant flange
- timing belt/waterpump (every 100k)
- Rust
Those are the common issues/attention areas, the rest is just the typical things you should expect to break on a 15 year old Audi, which is everything.