My advice is to stay away from an S80. We have a 99 S80T6 which has given us nothing but trouble. Do a search on this board and you will find many other complaints. They are priced good for a reason I believe and that is to get rid of them. I tried to sell mine but had no interest, probably asking too much or buyers already know the troublesome history of these cars. Still exploring ways to dump it without losing too much.
I've heard the later models are better but I'm not staying with Volvo to find out. This was our first and last Volvo.
Have you looked at Bimmer's?
If you're really interested in economy and reliablity, then Japanese (HONDA IMHO) is the way to go. I've got 174 K on my Accord with very little trouble. If you did this, you could keep your 944 when you need a fix!