Wash and clay the car to remove contaminants and take your pick of polishes for what will be adequate for your current condition of your paint and a DA. Might as well do little correction while you're at it. You'll need a decent wax afterward of course. Keeping it waxed will keep the spots away. You'll never avoid rain and spotting, you can only make it easier to remove them, when they're sitting on top of a good layer of wax or sealant.
Porter cable 7424XP is a good unit for a beginner and pick up some of the lake country or chemical guys pads. I'd suggest some 5.5" and if you want to make b and d pillars and along roof edge easy get a backing plate for 3" pads as well. They're also great for glass. Zaino makes a glass polish that works wonders and will leave it silky smooth and clear.
The distilled or deionized water rinse is the best advice if you've just washed. You can let it virtually air dry with no spots. If its windy or you live in a dusty area youd still benefit from drying.
You can pick up a decent "portable"...will hook to garden hose...DI system from many of the aquarium hobby vendors. Airwaterice and spectrapure make great units. Spectrapure is rather high end and likely far more than necessary for this application but they're quality and efficiency is second to none. I used to co own a detailing business and have also been into the reefing hobby for many years and it has come in handy countless times. I've also owned about a half a dozen black cars, the expedition is actually the first car since 2006 that wasnt black.
The car in the pic below was done w a 7424, chemical guys hex logic 7.5" pads...orange, green, blue, black, red and finished with Glossworkz glaze, Jetseal 109, and topped w 2 coats of Pete's 53 paste wax. Polishes used were the V34, V36, V38. They sell a nice sample kit w all of the polishes in 4oz bottles for like $20. Just the right size to knock out an expedition.