The 3.0 TDi is a great engine, my favourite. I've had all versions back to the A6 Allroad, up to the S4 Avant, and currently have an A8 TDI, which is ridiculously economical on a run.
My two bits of advice are that with high mileage, the EGR cooler can need swapping, and the fuel injectors can go bad (6 of at £200/250 each for the correct ones) but my A6 didn't need any of these until 158k on the clock. Gearbox oil change is definitely worth doing on an auto (lots of oil!) But your manual is less critical.
I have religiously used Miller's Diesel Eco Power Max for years, adding a bit every few tanks, and never had an issue with anything 'mechanical'. A lot of people swear by it, and others will say its snake oil, but I agree with the former.
They are solid engines - I had a Revo remap done on the Allroad at 120,000 miles which brought it safely up from 240bhp to the 286 /282 I now have in my A8.
Enjoy !