I just replaced both O2 sensors and didn't pose much of a problem. I ordered them from FCP Groton and it took me about 1.5 hours to do it in my garage. The rich condition might just be the computer trying to compensate for the O2 sensors but that is out of my expertise.
It would be very unusual to hear of a turbo going on one of these cars but not impossible. The oil you are talking about is normal.
I will let you in on one thing I have learned. Volvo's, like other european cars only go in the shop twice a year. One service will cost about $50. The other will inevitably cost $500!
Good Luck!