5 years or 120k whichever comes first I think for Cambelt. Definitely make sure service history is good. And oil changes for gearbox and quattro system. Check out buyers guides on YouTube
3.0tdi is a good engine. Look out for timing chain rattle and the injectors can have issues.
There's also the 2.7tdi in the c6 you could consider which is similar to the 3.0tdi.
Check service history.
Also have a look at YouTube buyers guides.