With that much mileage, I would also recommend an oil filter and oil change, a THOROUGH cleaning of the interior of the windows (dirty windows is one of the biggest reason for fog ups, in fact, most clean windows are much less likely to fog up), new wiper blades if you need them and a an extra set all around (excluding the headlight), cabin air filter should be replaced (the more it is clogged, the weaker the defrost and air conditioning), a brake fluid flush, replace the fuel filter, have the suspension looked over, in particular tie rod, anti-roll bar, anti-roll bar link rod, spring seat/strut mount, make sure succeptible seals are checked and replaced if necessary (oil filler cap, dip stick (oil and ATF, etc), I would also recommend a VERY good exterior wash and wax with a good long lasting wax (since you are probably looking for more protection then shine, go with polymer waxes), replace battery if it has not been relaced in the last two years (the oe Volvo battery is known for reading well right up to the point at which it fails), how long until your timing belt is due? it may be time to replace, it would be a good idea to flush and fill the cooling system if not done in the last year, is your AC cold? it may be time to get a recharge, not only for comforting AC, but also because a properly operating AC system puts less of a strain on the car and helps to clear windows,

As for things to bring, an extra pair of head light bulbs (9006), tail light bulbs, turn signal bulbs, an extra quart of engine oil (to top up small leaks/consumption), 2 oil filters, an air filter, a quality ratchet with 19mm deep socket with a cheater/breaker bar, some foldable wheel stops, road side flares, reflective triangle, a large jug of distilled water, a roll of shop towels, a couple bottles of a good fuel injector cleaner, an emergency blanket, energy bars, bottled water (large bottles are more efficient and are more easily made into tools (i.e. scoops, funnels, etc) when you really need them), a bottle of concentrated washer fluid (high alcohol and detergent content, when mixed with water, forms regular windshield washer fluid with somewhat good winter capabilities), three good quality flashlights (preferably atleast a tri-D-cell preferably quad Maglite, another with the capability to stand on its on and another smaller penstyle light for interior use such as a dual-AA Maglite), an accurate tire pressure guage (dial ones are more accurate), wet wipes, aspirin, tylenol, bandages, disinfectant, wraps, a high quality multi tool (leatherman), good sunglasses for the driver (Serengeti and Ray-Ban make some very good driving glasses), atleast 2 glass holders that attach to the visor if you require corrective eye wear, 1 if you don't, work gloves, jumper cables, a manual for the car, a CELL PHONE with access throughout North America, join CAA/AAA (they will make custom made maps for you from anywhere in North America to any destination within North America (maybe not in Mexico) for free, as long as you are a member which can be a life saver, and pays for the membership all on its own), put CAA/AAA's number on the cell phone as well as your destination's cell phone for local directions, a car charger for the cell phone, an AC adapter for the cell phone to charge at hotels/place to stay, duct tape, electrical tape, STRONG zip ties, a can of aerosol all-purpose lubricant (a good one is Slick 50's), a torque wrench, a set of metric sockets, a big strong flat head screwdriver (atleast a foot long), a vice grip, some miscellaneous screws, atleast 10 plastic bags, pencils, pens, notepad, zip lock bags, check to make sure you have extra fuses at the fuse box, nitride gloves (the blue latex-like ones, that are resistant to puncture and oil, which the latex are not), lip balm, skin moisturizer, sun screen, a good cup holder, anti-nausent pills, anti-diarrhea pills, you'll need both going south of the border), emergency rain ponchos, strong double sided foam tape (3M makes a good one). I can easily name more stuff, but I think you get the idea.

Delicate things like bulbs, should be stored appropriately. Most of the things I have listed can be stored in the rear side compartments that straddle the main opening for the spare tire, and above the spare tire under the floor board. Everything except for the distilled water should be able to fit under the boards. Obviously, things like the flashlight, cell phone, maps, shop towels, wet wipes, plastic bags, zip lock bags, emergency food, medicine and water, emergency blanket, first aid, tire pressure gage, lotion, first aid, etc, should be stored above the boards and readily accessible.

Also, learn how to read and clear codes on your car. A couple codes, if not read and cleared will not allow you to start your car, especially tranny codes. And since the reason for the code is often of vital importance, bring the manual with you.

A full size (205/50/16) spare will fit. I wouldn't worry about a spare fuel relay or spare seal for the fuel pump as these are VERY rare problems for the 850.

I may have probably scared you by listing all these things, but take it from someone that has done a number of these long distance trips (but not nearly this long), these things are LIFE savers. And all the stuff I mentioned about things to do before you leave, an ounce of prevention is worth a pound of cure.