Make sure you have your mechanic check the main v-belt, water pump, and especially the timing belt. The A6 uses an interference engine whereby disastrous results occur when that timing belt goes - like internal engine components crashing into eachother. Shouldbe fine as long as car was serviced regularly and doesn't normally need to be changed until 70-90k miles.
Make sure the car is up to date on all service campaigns - just get the VIN and call your local audi dealer or audi corporate customer service. They can look it up in thier computer for free. Anything that has not been done will be fixed for free.
Also I highly recommend that you get a carfax - many car dealers will offer this to you for free but if this is not the case I would pay for it myself. It'll give you tons of useful info about the car history.
Get the car on a lift and check for any leaks. Be sure to check for uneven tire wear as this can be indicative of suspension problems. And finally, be sure to test every electrical feature on the car (there are alot so this can take some time) since Audis have a rather spotty reputation for electrical system reliability.
Hope this helps - I recently purchased a 2000 A6 Avant for my wife and we are very happy with it.
