About what year you looking for?

General opinions seem to be avoid 93 & 94 (don't know why, I have both).
95 good as last year for computer you can talk to without special scan tools.
96, 97, 98 best years of 850 style (98 new body but same under the cover).
1999-2001 avoid due to expensive ETM repair (although rumor has is Volvo will replace. ETM fails without warning and leaves you standed with $1000 repair needed).
2002 & up don't see as many issues yet.

Also - 850s require more maintenance then my 245 and not as bullet proof. But much higher on the comfort scale. I have no experience to comnpare with the 122 or 1800 for you. Sure others will.

If you find one post info and you'll get more detailed info. And as with any used car, get carfax.com report.