If you're going to use a grinder, I've had good luck with a 4 1/2" with a twisted wire cup (fewer projectiles than straight wire!) for rusty areas.
If you have a lot of fairly flat, open surfaces that involve simple paint removal, you may find 80 grit disc's on that grinder will be a lot easier! That grit will leave a good 'tooth' for filler/primer/paint to grab.
Either way, go slow and don't generate too much heat by staying in one place too long.
I've found that cup brushes have a nasty habit of developing an out-of-balance vibration that drives me nuts.....and beats the hell out of my hands.
Don't forget the mask and eye protection.....
