I've owned a lot of Volvo's inmy time and currently own 3, an amazon wagon, a 2003 S60 (same everything as the wagon you are considering) and a 2010 XC70. The 2010 is too new and the 68 is too old to compare, but the 2003 is just right. We've had no major problems at 170k. There was a neutral downshift issue and a turbo control module that needed replacing, but both took no time and almost no money (<$100 each). The suspension has sucked, but we drive in Houston which is hell. I've done new shocks and struts three times in 10yrs. Finally going up to the highest level stuff on the last rebuild (not OEM).
My folks have a 2001 s70 which has had almost no problems.
I think they are great cars and I think they last forever. I don't think it is fair to say anyone of them has some dark horse waiting to come and claim it at XX,ooo miles. I think there are bad cars in every brand and model, but a universally bad issue in any of the 9 volvo's I've owned, I've not seen.
If you do buy it, go ahead and have the T-Belt replaced (110K service) At this age it will die from age before miles. Also look at flushing the trans fluid, other than that drive it into the ground.
Also, don't take mechanic advice from folks that are not Volvo mechanics. Use Swedespeed.com to find a good non-dealer near you.