I think you are going to be very unhappy with the results.
320 "Fine" is good for around the house, sand and brush paint with latex finish... NOT an automotive finish. It's a good starting point if there's damage being repaired...
If you kept your receipt, take it all back.
If your center bezel is in good shape, (I'm guessing it is.) you won't REALLY need to sand it at all, just wipe it down
real good with a good cleaner, Prepsol, or other degreaser, even household rubbing alcohol will do in a pinch.
You don't want to hit the surface with any sandpaper courser than 600 grit if you don't have to. 1000 grit will scratch the surface more than enough for paint to stick. A fine Scotchbright pad will work too. (< maybe best!) Your best finish will come from NOT sanding at all. Just getting it real good and clean, and dry. Also get a "tack cloth" to do your last, dust free wipe before painting.
You need to go to an Auto Parts store, Auto Zone, O'Reilly or Advance will do, but you need an auto parts store. They'll have the paint, masking tape and the abrasive matterials you'll want/need. You'll have a much better color selection too.
Don't worry about the parts you are painting being plastic, they're already painted, you're just repainting them. Should be fine with a recoat.
Do it right the first time. Take your time.
DON'T USE WHAT YOU HAVE!!!
Here's your shopping list:
Scothbright pad (Fine) or 1000 wet or dry sandpaper.
Prepsol or other pre paint degreaser
Tack Cloth/rag
Primer (Optional)
Color of paint desired
Good masking tape.