Andrew,

First and formost, with any vehicle (volvo or not)...the mechanical condition is going to ultimately determain the life of the car. It is most definatly good to find a car with full service records (as you have). Even if a car has been regularly serviced, it is possible that the owner does not take the best care of it, and that's really a judgement call on what you think of the seller. More often than not someone that's willing to always pay to have their car serviced on schedule takes good care of the vehicle.
Traditionally Volvo has made part of their name making indestructable cars, I've seen serveral 240s with 300,000 or more miles on them (and of course lets mention Irv Gorden's 2 million mile P1800). As far as the 850, through my experience and opinion, that if it is well cared for, and you don't mind perhaps having to pay once or twice over that period for a major fix (e.g. AC evaporator) you'll be fine taking an 850 to 250 or 300k. In my opinion it still matters most how well the car has been taken care of, I have friends who've put 200k on Mustang 5.0s, Jeeps, and Mopar Mini-Vans. Imagine how much further the sturdy engineering of the volvo would allow it to go with proper maintance.

You may want to post a question about the logevity of the turbo unit. I know sometimes there have been trouble with them in Volvos in the past, but I havn't heard too many stories of 850 owners with high millage needing replacement.

I talk to much =)
Brandon