need some pre-painting advice, how much prep is required on "hidden" parts? Options

[914 RZ-1]

I've decided I want to paint my car myself. I'm not going for some award-winning concourse-quality car with a $10,000 paint job, just something that looks better than the beater I currently have. The car was in an accident long ago and repaired well, but it's not 100% perfect, so it doesn't make sense to me to spend tons of money on it. I'd rather drive it and enjoy it. That said, it's still a nice car and runs great and looks good.

I plan to spend most of my time finishing the parts of the car that will show and less time on the parts that are usually hidden: under the hoods, the door jambs, inside the trunks, etc.

What's the minimum I would need to do to get these surfaces ready for paint? Scuff with Scotch-Brite? Sand with 220 or some other grit? Sand blast? Is it better to use power tools or do it by hand?

The car was originally metallic silver. It is currently white (engine bay, area by fuel tank are still silver). I plan to paint it a solid (not metallic) gray that closely matches the silver.