OEM Turbo plugs are $28.49 from volvoparts.com (a volvo dealer).
Their are other online discounters, too. I've always used the
OEM and they last 30K and have not caused any problems.

Rear main seals fail fairly often. Most common in non-turbo engines
when the flame trap plugs (no flame trap on turbo motors).

I'd keep an eye on the O2 sensor -- it can affect milage and engine
performance if it fails.

The owners manual has a pretty good checklist. One thing it recommends
is a brake fluid flush every two years. The fluid absorbs moisture
(by design) and this can shorten the life of the (expensive) ABS
components. I'd also chack the pads and rotors for wear. The
rotor has the minimum size stamped on it. I think Bay 13 has the
numbers, too.

If you don't have the manual you can order the
glove box documentation from http://www.volvotechinfo.com/.

This tool makes brake flush/bleeding easy: http://www.motiveproducts.com/

You might also consider alldatadiy.com. For $25/year they give
you the technical manuals and service bulletins. This has
proven worth the money for me.

My wife has a 95 855 GLT and I purchased a 98 S70 T5M (so I voted
with my wallet) and I don't think you made a mistake at all.