Is buying a 2001 Audi TT with 99k miles a good idea? experiences?

Your main concern is going to be the timing belt. It needs to be replaced every 60K miles. Check with the current owner about when/if they had it replaced. The second concern would be the ignition coils. Make sure the owner had these replaced as well (there was a recall issued by Audi a few years ago because of ignition coils going bad).

Just keep in mind that if something does go wrong, it may cost more to fix because it's a European car and thus the cost of parts can be affected by the dollar/euro exchange rate.

Cars usually "go" because they haven't been maintained. Do a comprehensive check on this vehicle's maintenance history. Find out if the timing belt service has been done. This includes more than just the timing belt, by the way. Pay the money for a carfax report, it's worth it to find out if you are going to make an expensive mistake. Turbochargers allow engines to have great performance but they are extremely hard on engine oil. Make sure the previous owner(s) changed the oil & oil filter at regular intervals with the appropriate Audi specification full-synthetic oil. Use NADA and KBB sites to get an idea of the car's value should be. Don't overpay for a car especially if it hasn't been well maintained.

Audi's are premium cars that require premium gas, have expensive replacement parts, and come at a premium price. But once you drive one, you'll never want to own another brand. Do proper maintenance, and these cars will reward you for a long time.

that is not completely true