One major thing to check is the rear diff - a Mate has one and it has been in to Audi due to an oil leak. Major job stopping leak and if you cannot stop the leak its new diff time. not a massive concern for Audi but there have been enough cases to be notable. His was the active diff - worth asking the right questions regarding the rest of the car too. Service history a must. S-line seats are not so bad - some would prefer them to the RS Wing Backs that can be too supportive - especially for the larger driver / passenger. Any Audi model that has an RS derivative will suit Wing backs. Note there may be some electrical harness issues if the specs of the seats you have vary from the prospective seats.