Started looking at epoxy, and wondering what the quantity I should get?

For this a quart of SPI epoxy and catalyst is probably good. That gives you 2 quarts sprayable. From what I recall it's also UV stable, but I'd topcoat it.

I don't think I'd try spraying it under, say, 55* or so though.

I still say that I'd get it blasted first, no matter what finish you go with. Adhesion will be worlds better if the base coast is painted on a rougher surface, especially if you're not worried about it being shiny.