The big things are: suspension, cambelt and gearbox.
The air suspension is well known for its failings. The inflatable airbags tear (so they look lop-sided when parked and settled) and the compressor has to work overtime to keep up and wears out. Ideally, make sure any car you view has had lots of time to settle (ideally overnight) so you can tell whether there are problems. And make sure it rises and falls as it should using the button on the dash. Ideally you want evidence that it's already (recently!) been replaced. Not cheap to fix.
Cambelt change is a big job because the front of the car has to come off. Main dealers charge about a grand for belt and waterpump change. So worth making sure it's been done.
And higher mileage Audi autoboxes on Quattros are known to have problems at higher mileages if they haven't had fluid changes.