As mentioned, a build up of engine sludge due to longlife servicing and the use of the wrong type of oils is the main issue with 1.8T's. "If" you buy it it's well worth you dropping the sump, and cleaning/replacing the oil pick up as plenty of engines have died through oil starvation with this. Then you'll know you've a good car which will last and last with the regular quality oil changes.
Otherwise, check the coil pack and plenum chamber gromet recall has been done. Other issues are:
1. Makes sure all doors lock/speakers work, wires break at the hinge point.
2. Check the glovebox lid.
3. Make sure the wipers aren't 'slow'.
4. Aircon works and as you direct the flow it comes out of all selected vents.
5. Make sure the engine undertray isn't missing or hung off.
I'm sure there's more, none of the above should put you off, but forewarned is forearmed
Check the clutch, if its slipping in high gears then your looking at £649 fitted at audi, also check that the timing belt and water pump have been done, should be done at 70k or every 5 yrs so yours should be due its second one, think these are about £350 at audi.
As above check for sludge and regular oil changes i do mine either every 5k or 6mths which ever comes first and make sure the correct oil has been used. Everything else is just general wear and tear, brakes can be get a bit of a hammering especially if the engine has been remapped and if you can get hold of a vagcom lead or know someone who has one get the car scanned it might throw up some hidden faults.
These are awesome cars and the best engine good luck with your purchase...