I did it for peace of mind. Regular oil changes/flushes etc.
I was doing minimum once a year. More if I were doing road trips.
Got to get my engine rebuilt now.
I'd recommend you look at getting a stainless steel straight through exhaust. Powerflow are reasonable and I know people who have used them and been pleased with the results
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.