There are 2.5 5 cylinders and there are 2.5 V6s.
When I last drove a V6 TDI I was massively underwhelmed.
Doesn't have the gutsy delivery of the 4-banger TDI.
2.5 5-banger though. Go on. It'd be a bit old, but hey, what's not to like about the older A6 Avants?
Go look on Mobile.de and think about nipping over to Germany to pick something up - huge range of interesting stuff out there.