I would consider a good go-over of the engine, unless you have paperwork to see when items were done. Some things I would consider, most being DIY, and not all need to be done ASAP, but will give you peace of mind for years:

1) brake fluid flush - have a good garage do it, don't risk an accident. Flush is cheaper than ABS pump, probably wasn't done before you bought it.
2) Hoses - upper and lower - cheap too. Get a new thermostat too while you're doing the hoses and flush the coolant.
3) Check/change serpentine belt - IPD or others sell a tool to make it easy, if it breaks you're in trouble on the road.
4) Check battery age - how old? Four years is about it for a Volvo, get one at Costco for about 40 bucks.
5) Fuel filter - about 25 bucks, DIY job, watch out for fuel under pressure when removing old one though.
6) Air filter/flame trap. Definitely check flame trap, leading cause of RMS failure.
7) Plugs, wires, cap and rotor - all DIY, be sure to clean plug ports to prevent stuff falling in there when removing plugs - use shop vac.
8) Top motor mount - if lose, replace with solid style plastic type - DIY.

Those are some simple things to check, replace to keep the 850 on the road for a few years at least. Volvos like regular care, then its not too much money.

I would check to see if the car has a cabin filter - under the plastic cowl on passenger side. See if its there, or if stuff has fallen down in there. It seems that the filter protects the AC, at least that's the rumor. I added the filter kit which cost about 35 bucks.