Moin,
buy the parts at your dealer. The price-difference doesn´t make up for the delivery-cost.
The damper and the main screw is available from Audi only. Complete cost approx 350,- - 390,-€. (Belt, damper, 3 rolls, main screw, water pump).
If you wanna keep the prices as low as possible there remains e-bay (but only from good dealers).
Concerning the belt-renewal-timing:
every 90.000 - 120.000 km (depending on the motor), or latest after 5-6 years.
