my experience...and this is ONLY my personal experience (and my families as we have had volvos since the 144)

144 - bullet proof, but rusts (eventually)- exceptional engine, this made the volvo reputation

245 - fairly bullet proof, unfortunately has the b230 in later years....see comments below about b230G

744/5 - some good, some not (any b230 that hasnt had the right coolant in is a nightmare...), rust worse than a 144, b230G a STUPID inlet design

940t - great car, if only they were reliable and didnt cost a fortune to keep running, that bloody b230 AGAIN...

854 - 2.5 20 valve - brilliant, get in, drive, service...DO change the tranny fluid, eats brakes (discs) but stops well so you get what you pay for...no harder to work on than a 2/7/9 IMHO

later cars have fuel injection/electronics...

for simplicity and reliablity 144/244 (but any that are old enough are also likely rusty). Remember the 850 is a quantum level better in design and manufacturing practice, but DOES require you to keep on top of the maintenance...