Well, IMHO unless you're able to fix everything that fails on a now old 240 yourself, you won't be overjoyed with it either. Old Volvos are great for Volvo mechanics but can be a bummer for those who own them and need to pay for every repair and all old Volvos need something, some more than others.
As for selling an 850 or anything else, tell them NOTHING. "It's been a great car but".....make up something believeable BUT never the total truth. Do NOT tell them to come here first or tell them anything about opinions like "only factory trained Volvo techs can fix it" (even if so).
You want to sell so to the new buyer, it's buyer beware. Don't tell them too much or you'll never sell it for a decent price. My opinion of coarse.