Depending on the maintenance done before, its a good idea to do work now while the weather is good. Fluids, especially the coolant, should be changed if they are due according to the schedule. I would change the fluid if in doubt, and keep records afterwards. The coolant is critical, for 50/50 mix and with the aluminum engine parts it touches. A few bottles of coolant are not expensive. Volvo specifies their brand, but there are other brands that are compatible. Check the manual for the type to buy and avoid the others.

I would check the tires for tread depth. Ideally, a set of winter tires on rims would be great, but since you're in school, that's probably not an option. Volvo recommends chains on the front wheels only, so you may consider a set.

I would buy RainX winter windshield fluid, as it works well and coats the windshield when used. Wiper blades should be checked. You may want to get winder blades with rubber boots covering the metal section to prevent ice buildup.

Your fuel filter is probably original, and that is an easy DIY job to replace. Be careful of some gas escaping under pressure when undoing the hose though.
Air filter should be checked/replaced if necessary. Battery - how old? Heat kills batteries more than cold, but in the winter you use a lot of elect. power too. Spark plugs are cheap and easy to do - go to the dealer and get the 3 prong replacements - no gapping. How about the timing belt? I hope it was done around 70K or you need it done ASAP! The accessory belt should be checked/replaced too.

If the 850 was well cared for you should be in good shape. Like any car at 100,000 miles, some things need to be changed for maintenance. I've done the above and never been stranded. I live in Michigan, which didn't get a lot of snow, but had a ton of single degree days last winter.