One thing to be careful of is the suspension. Audi used a complicated adaptive system (not air suspension) and when it goes wrong it's expensive to put right. The system is designed to keep the car flat through the corners.
You'll find that some rs6's have been converted to coilovers as it's cheaper than fixing the Audi system.
Id advise finding one with good service history (ideally full) as the rs6 can be costly to fix if it goes wrong (i.e. The gearbox). Oh and low mileage as poss.
Check for crash damage and damage to the rear arches. Rear arches in the a6 c5 aren't known to rust unless a repair has been carried out. Body parts can also be expensive and with regard to body work Audis and VWs from this era also suffered from the front wings rusting at the top of the arch.
Never seen it on an rs6 mind but the a6 c5, a4, golf mk4, bora etc were all affected.
Oh and make sure all electrical components work. For example it can cost over a £1000 to fix the aircon if the pumps/condensers etc are broken.
they are cracking cars and will be a future classic :)
hope iv helped :)
cheers
steve