I've got the 2000 A6 and it runs like a champ.
These cars have outstanding interiors and just solid engineering all around. For the money you really can't get a better car. I'm pretty familiar with the 06 model and it is a fine car as well.
Mine has 160,000 miles on it and has given me 100% solid performance for the past 10 years.
It is on a top tier maintenance program so the timing belt has been changed twice, along with starter, alternator, all suspension parts, wheel bearings, windshield, wheels, transmission seals, headlights and brake lights, exhaust system and some other stuff.
None of it was really shot, the car is just kept to showroom condition since it is one of our favorites so it gets a lot good preventative maintenance work.
If you want however, you may want to take a look at an 02 or 03 A8. Those are the newer style and those cars have a 100% aluminum body, hood, doors, frame, roof, everything. The A8 is an absolutely fantastic car that is very safe and a great long range cruiser. The A6 is still nice, but a little more dough can get you one of those A8, and most are in very good condition.
But like family tree says, check that engine and trans. They have very few problems, but a good mechanic inspection is cheap insurance.