2500 or 3000 grit. you dont want any thing to aggressive, even 2500 is going to remove the clear pretty fast. with a light pressure, just let the paper slide over the paint.
you dont want to sand it like its wood, and stay off the edges or you can cut through the paint, be very light when sanding on those areas.
id not remove all the clear coat thats still stuck, the sanding is only for a mechanical bite for the clear coat to stick on there. any of the light scuffing you see the new clear coat will cover up like it was never there.
id search youtube for some how to vids, what i did when i first took on a larger paint job, aka both bumpers off my regal. theres all sorts of tricks you can do along the way to make life easier on your self as far as taping things off and also gain the confidence to just dive in. cause its really easy work, just takes some time.