I'm in the same boat looking for a 3.0l quattro.
Are you looking for an automatic or manual? If automatic look for the gearbox services and oil being changed at the appropriate intervals. The stronic gearbox is particularly expensive to replace/repair.
The 3.0ls have a cam chain so listen for a rattling on start up, this would indicate a potential problem with the cam chain itself
Check all of the electronics, again they are expensive to fix if they go wrong.
The window regulator's are a weak point and tend to go wrong, so check the windows open and close properly.
Good luck finding one, I'm beginning to lose heart.
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