The turbos are not really an issue. Transmission and air conditioning
are the big items. Check to see if transmission fluid has been changed
around 30K. Volvo says don't, but it is a good idea. A common problem
with transmission is called the PNP switch and costs about $300 to fix.

Air conditioning is a bigger issue. Many cars leak refrigerant. Fixing
costs well over $1000. A crafty re-seller will recharge the AC so
it is working but a slow leak would not be uncommon. Recharging is
maybe $150 and gets you a year or two; but eventually they need servicing.

Just noticed you already drive an 850, so maybe you are familier with
these items. We own a 95 850 and a 98 S70 and overall, they are similar
in reliability. I would not prefer a 98 over 99 or 00 but many people
will avoid a 98 since it is first year (might get a beteer price on 98s).

This site has many common problems and their solution:

http://volvospeed.com/maintence.htm