Price definitely high (unless sub 100k mi) and generally not a car that one buys looking for cheap motoring. That being said, if well maintained driven regularly then they are pretty reliable.
Still, not going to bash your decision as these 850s are great cars, they look fantastic in my eyes - not that I'm biased or anything!
GLT with the winter pack so not a bad spec - depending on how much original owner decided to cost cut anyway. I see the steelies which probably means they cost cut a lot! GLTs should come with half-vinyl and half-cloth seats at a minimum - pretty comfy but also easy enough to swap for full leather options, full interior sets normally sell for £100 of so. If you did want to upgrade the wheels you can find OE alloys ranging from £60 a set to £150 depending on condition. The 15" Cetus alloys would have been the normal spec on GLTs and the 15" size makes finding tyres slightly easier than with the 16" alloys that are available. Avoid anything 17" (have to run bizarre tyre sizes and still rub on full lock) or anything more than 7J (simply don't fit up front on the 850s).
If your heated seats don't work it will be the wires to the thermostat (repairable) or a break in the heating element (eBay replacement).
If A/C doesn't work its probably because the compressor clutch gap is too large (ziptie fix), or the evaporator is holed.
If you don't have a pollen filter find one. Can buy aftermarket housing for £15 or so or genuine used for £10-15; the advantage of the pollen filter (aside from the obvious) is it stops debris from getting into the back of the blower motor and evaporator. Remove the two passenger side T25 screws on the scuttle panel to check.
Definitely replace brake fluid if it hasn't been done in the last two years or the car has been sitting. I had to rebuild three calipers because of this scenario leading to brake fluid absorbing lots of water and then rusting calipers from the inside out.
Definitely check the PCV or make sure you have receipts for it being done within the last 50k. If not you'll be doing it very soon and it's a real pain - not that cheap either. Two ways to check, either by looking for smoke coming out of the dipstick tube whilst idling at temp, or by putting a rubber glove over the oil filler hole. If the glove inflates whilst the engine is running then you have a PCV issue and need to service it. If it gets sucked in then you have a good system. Obviously you need to secure the glove so it doesn't get eaten!
10W40 is the correct grade oil, don't deviate! Can go fully synth or semi-. Doesn't really matter as long as it meets the right specs and is the correct grade. Some claim that the high-mileage specific fully synth oils have helped with oil consumption and reducing oil leaks. If you get semi-synthetic then Volvos official service interval for the oil is 10k - but most oil companies would recommend changing every 5k (and so would I), fully-synth is perfectly fine at 10k intervals.
