i'm a weekend novice bodyworker, but my experience at home and at bodyshop class was 80 grit seems something to start with, aggressive enough but won't put deep scratches in the metal like a 40 grit disk.
i'll defer to the experienced bodyworkers here, but i kinda take a strip it to bare metal approach. preserving original baked on primer might be fine. but i go to bare metal with a let's do it all approach. i also live by an epoxy primer on bare metal, then plastic filler for bodywork, then epoxy primer over filler, then the leveling primer coats for which to block and prep for seal then topcoat paint. Keep this in mind if you need to fix any dings, which i'm guessing are inevitable no matter how original and preserved your car may be. hell, I found filler under my hood from the factory (it was a nickel sized pin point dent that must have happened at the factory because it was underneath the original forest green paint.
Kevin