No idea where to begin!

These guys are right on. If you are stripping the car down to bare metal, 320 will take forever. My suggestion with the 320 is if you plan on keeping some of the old finish as a substrate. If you can't afford or don't have access to media blasting equipment, I would suggest buying a D/A sander and strip it with 36/80 grit. When I strip a whole car to bare metal, I use a chemical stripper. It's nasty stuff, but it works great. Wear long sleeves, rubber gloves, safety glasses and do it outside if possible. KleenStrip Aircraft Remover is a great way to strip paint. Get a good metal scraper and some steel wool, this stuff will take it right off. Start with one fender or a door to get the gist of how it works. It's messy, but effective.