Paint sticks well enough to these these resin parts unlike the glossy finish on 3d printed plastic parts. Since you won't be handling/scratching the parts much after they are painted, I don't think you really need to use primer (many would disagree), but if you want to, I suggest a can of flat/matte Rustoleum or Krylon primer (not glossy).
Resin parts are more brittle than plastic and can shatter if you attempt cutting them. Sawing, drilling, or grinding would work better than cutting. If you're going to cut anyway, your best bet is to dunk the part in very hot water first. Cold resin parts shatter easily. Hot resin parts are softer and have much more bend to them before they break.