just carry in your car a gallon of water or coolant (if you have an empty coolant jug, use that), a set of jumper cables, and MAYBE an extra headlight bulb. If you have a serpentine belt system, a $25 belt can be good insurance...it is ONE belt instead of a slew of them under the hood...so if it breaks, you not only lose emission control, but alternator, power steering, water pump, etc...
Look at it like this:
Electric...Jumper cables, Keep your battery filled if it isn't maintenance free. Accessory Belt.
Engine Coolant...water, but coolant best. if you are out of water, you can pee in it. Have an extra thermostat and gasket goop. SOMEONE will be able to change it if you can't...at least you'll have the part. Roll of duct tape for hose leaks.
Engine Lube...just keep ALL your fluid levels up at home. Maybe carry a quart of oil.
Tires...keep them inflated, keep a gauge handy. Fix a flat really sucks, and a tire shop will KILL YOU if you don't tell the tech you have fixaflat in there...when hie inflates it and yanks the stem he'll get a face/hairfull of the sh*t and you can't wash it out of hair.
Fuel...I take it you aren't a mechanic...I would carry a fuel filter, but that may be a bit involved if you don't know what you are doing. Carry a gas can in your car.
Now, you implied you want to be prepared not to have to buy parts replacements if they will be more expensive later...well, something to consider...don't buy NOW if prices are higher. Why buy oil (lube) if the price is higher now? Not that I recommend it, but if you check your oil level, and it is level, you can run your engine WAY WAY beyond the recommended 3,000 mile oil change. All I mean to say is DON'T sacrifice food, medicine, etc. to change your oil. Do it WHEN YOU CAN (in a SHTF scenario). Also, make damn sure you have water or water/coolant if you will be idling for long periods of time OR live somewhere like Texas, where rush hour traffic can leave you on the side of the road. You generally have to rupture an oil pan to run dry of engine lube, but a faulty $3 thermostat can stick and cause you to lose all your coolant, which can cause an engine to seize, warp your heads, etc...
Most of the dead cars I've seen are *out of gas, dead battery, busted belt, over heated (coolant, thermostat), flat tires.
Also know that if you are in TX and stranded on the highway, there is an emergency # on the back of your license for roadside assistance. Prolly a WTSHTF scenario lost cause, but good to know...and I bet other states are do the same. Hey, your tax dollars, why not use it!
Hole that helps, I know it was convoluted, but so are vehicles. PM me if there is anything in particular you need to know.