I dont think mileage is a massive factor as long as servicing has been done properly. There are some common faults with S3's but if they are looked after they should be few and far between and already sorted with usual maintenance.
Check for rust on the roof rails and down by the rear wheel arches aswell as the door rub strips. Check the headunit works properly, especially the volume control.
Check the digital display between the speedo/rev counter for missing pixels.
Might be worth buying one that has been remapped as you'll probably want this doing if you buy a standard one and it costs £300+
Check for broken rear springs, again very common.
Go through the service history with a fine tooth comb, it will be worth it in the end.
Obviously check against HPI and MOT database (free) and check cambelt services have been done properly with water pump.
Check temp guage works as thermostat and sender unit are common failures.
Have i missed anything? lol
Oh, try to get someone with VAG.com to go with you and check for fault codes.