If you don't have much history on the allroad, I'd suggest looking at the 4.2 A6. The 4.2 is quite a bit easier to deal with as far as maintenance costs are concerned.
I mean... With the allroad, you've got to deal with airbags that'll basically go out roughly every 50-60k unless it's got some aftermarket ones. Plus the compressor... Plus a slew of other things.
If you go to quattroworld, they've got an allroad buyer's guide that'll basically cover over what to look for.