I'm just finishing my '85 RX - welcome to Rotary Land.
Anyways, with the black paint take a close look to make sure it's not metallic - my '85 SE is, so I know it was in Mazda's Repertoire back then. And if the paint does have metal fleck I'm extremely wary of trying to get a good match with a spray can... I've tried and failed on my '84 RX.
With the repaint you'll want to take a VERY close look at the paint to see if it's bubbling at all from rust that wasn't properly fixed. On those cars the front edge of the roof right near the window is a problem spot, and around the rear hatch seems and key hole too. Also take your rear wheels off and inspect the inner and outer surfaces of your rear quarter panels.
If you do have rust to fix beware - it can be a very time-consuming proposition to do it right (as I can attest to) but if you want to undertake the project then I highly recommend POR-15.
Sorry I can't help with your actual questions - I'm still learning this all too.
