I'd echo Penfold's sentiments.
I rate them very highly, along with 99% of Volvo's. All I've had have been ultra reliable, spacious, well equipped and quick. I took a '96 Volvo 850 2.5 petrol estate in px against the Range Rover. It's done 208,000 miles and still drove really well, felt composed and was quick.
What year is the S80, which engine? I think the earliest S80's are 1998 on a 'S' plate. D5 engine is good but don't think that was introduced until 2001. The petrol T6 engine is great, it's a 2.9 turbo and goes very well. A few police forces have used them to chaffuer senior ranking officers around, some forces still use 2003/2004 S80's for this, admittedly only a few though.
Common fault I'm aware of, this isn't an expert guide, just from remembering issues with Volvo's and other people's experiences. The one thing that has cropped up has been the rear brakes and exhaust clips stretching that produce the old Volvo exhaust drone. You'll see plenty of 850's and V70 classic's with this issue. Only seen one S80 with the same problem.
All in all, good cars, I'd happily have one.