I think I've probably commented in every 850, V/S70, etc. thread over the past few years. I know there's a few threads on these already.
You'll be hard pressed to find a V70 with AWD & a manual that's not an R, 99% sure you won't though the P2 cars aren't my specialty. P2 are the later V70s, started in '01 and ended '07-ish. Mad_machine is right the first S/V70 was basically still an 850 with slightly rounder edges. I drive an 850, and have quite a few S70 parts on it.
There's nothing inherently "unreliable" with any of them, most all issues can be blamed on poor/deferred maintenance from the previous owners.
The non-turbos will give you the least issues with regards to maintenance, and aren't terribly under powered for the chassis. The 2.4l n/a still makes about 170 Hp. Many describe it as having driving characteristics similar to a larger 4 cyl, which makes since since it's a 5 cyl. Fuel economy ain't the greatest, I keep my foot in mine (non-turbo auto) and have been getting around 22 combined. Other people claim much better, but they obviously don't have my commute and heavy right foot.
Timing belt is probably the biggest killer, the engines are interference, bent valves will usually send the older cars to the scrap yard. It's not a hard job for an average GRMer, but can be time consuming. Volvo doesn't recommend replacing the water pump with each timing belt, but I and many others do. Don't skimp on parts, do everything, idler, tensioner, water pump, etc. every 5 years to 100K miles, which ever comes first. If it's not documented by previous owner, work the added cost into the budget of the car.
PCV systems can be a chore, it's a little more complicated than what's on the Jimmy. Some of the lines/oil separator can clog over time, then crank case pressures build up and cause oil leaks as seals are blown. Easy way to check is pull the dipstick out a bit with car running & up to temp. A little smoke is acceptable, a visible plume probably means it needs a PCV refresh. The parts aren't that expensive, but it's labor intensive, the intake manifold must be removed to replace most of the components.
The P80 cars (850 & S/V70 before '01) have been known to have A/C evap core issues. Some of the cars came from the factory without a cabin filter, years and years of dirt, pine needles, pollen, etc. build up on the evaporator, eventually they'll leak and the A/C won't blow cold/stay charged. If the A/C doesn't work, and there's not a cabin filter in place, I probably wouldn't buy the car. The evap core requires removal of dang near the entire front interior of the car to replace it. Check for cabin filter on the P/S cowl between the hood and window. You'll need to remove a couple torx screws to get under the plastic cowl and check. At least on the P80 cars. If it's an older car and the A/C still works, I probably wouldn't sweat it, it's either been fixed, or was never exposed to the conditions blamed for the problem in the first place.
As for the Volvo AWD, the later P2 cars are the one to get. Haldex get's a bad wrap, but it's a significantly better system than Volvo's earlier attempts. The early versions' bevel gear transfer case is susceptible to mixed/differing tire size/wear issues, and oil starvation. It doesn't hold much oil in the first place, and most don't catch a leak early enough to do anything about it.
You may notice when looking at used early AWD cars, that they ain't AWD anymore. Some one has removed the rear drive shaft, and using the car as FWD.
Volvo makes the most comfortable seats of just about any car make I've sat in, so they've got that going for them too.
I think that covers the worst of it. Happy to answer any questions. I've wrenched on 850, S70, V70, S80 T6, XC90 as they all are/have been in the family.
