Hi :) i never had a b6 but i have 2006 a6 3.0tdi quattro and there are similarities
The 2005 3.0tdi ( engine code BMK) can suffer from cam chain failure. Usualy the top tensioners that are failing first causing a distinctive metalic rattle for 1-2 seconds on cold and warm start. To fix you would be looking around £2k , and if the cam chain fails then your only option to replace engine . This was fixed in late 2006 ASB 3.0tdi
Tiptronic gearbox ( with i belive this car has ) is mostly bullet proof but can suffer torque conventer issues. Look for rev surges on engine breaking and gear slip on high revs
A vag com (or other) scan for faults is very good idea.
Front and back subframe can go rusty quickly
Other then that the car will have its usual weakspots. Have a look on youtube, there are plenty of reviews