I've had one for three years now. There are no major faults with them, but they are high maintenance cars. You can't run them on the cheap.
Engines are strong, but observe the basic rules. Change the oil regularly, and don't rev it hard until the engine is at full temp (by that I mean the oil temp, not just water temp). You've rightly picked up on the cambelt (maximum mileage between changes is 70K).
The Quattro system is pretty reliable, but they can suffer from an oil leak from the rear output shaft at the back of the gearbox. Front suspension control arms should have been renewed under recall. If they haven't ask why not and negotiate a discount.
As far as everything else is concerned, the normal rules apply. Condition and documented history are everything. Prices will depend on age, but there are plenty of price guide services on the net.
Don't use Audi franchised dealers. Any 2.8Q you will buy will be a minimum of five years old, so well outside the warranty period. Find yourself a good local independent garage with a strong reputation or, depending where you are in the country, be prepared to travel to one of the well known non-franchised Audi specialists like Fontain (Buckinghamshire) or Kim Collins (West Sussex).
Hope this helps.