Entwood was really impressed with the towing ability of his A6 3.0 Tdi Allroad.

I have the A6 Avant, which is a large comfortable cruiser but handles remarkably well when pushing on.
If you look at a 3.0 Tdi, be aware that they have a cam chain (3 actually), so if it needs a new cam chain or tensioners, the engine will have to come out as the chains are at the back of the engine. It's an eye watering expensive job.

It supposed to be a lifetime chain(s) but some have them checked/changed anywhere between 80K and 120K miles, so ask the seller to prove the history if it's a high mileage example.
Only the RS and BiTdi get the 8 speed proper auto box, the others get the 7 speed S-Tronic/Tiptronic/Multitronic box. The MMI/NAV system speed isn't a patch on bmw software but it's still a nice comfortable place to be.

I've no experience of A4 so can't comment.