I have an S80, albeit only the petrol model. It has hauled our lump of a caravan all over Europe without a hitch and will be doing the same again next year. I get 20mpg towing. The diesel has more grunt than mine and will tow anything, just about. The diesel will give 25mpg towing as an average, solo is of course around 40 mpg. It is also one of the most relaxing cars to drive especially in auto form. The boot is huge and the rear seat folds down giving dare I say it, as much if not more space than in most 4x4's.
The only weakness, if you like, is the front suspension. Bushes usually need replaced at about 3 years as the weight is pretty heavy.
Just ensure that you get SE spec and a full Volvo service history. Also look at fitting Nivomat shocks to the rear, it makes all the difference for towing. A Volvo detacheable tow ball is also nice, but the total package will cost you around