urethane is what new cars are painted with, a good quality urethane sprays well and covers well. Done right and it should out live most of us.
I like epoxy primer, the spots that have rust will need to be sanded to bare metal so that will require primer, probably will need a sealer coat to get an even surface to put the color coat on.
My choice would be epoxy as primer on bare metal and for the sealer coat ( basically primer with a little more thinner) it also helps promote adhesion of the color coat.
Paint wise you have the choice of single stage or base and top coat, (color and clear).
I use single stage and the shoot a couple of coats of clear on top of that for extra durability.
New cars are painted with waterborne paint. They require completely different guns, a different shop setup, and are hard to get to look good, especially if you've never painted with waterborne.
Use a base clear in urethane. You'll end up with a better finish in the long run, and it will be more durable. Like has been already said, prep is key to proper adhesion.
Also, was the existing paint done over the original paint, and was it completely stripped when it was painted? Sometimes the original lead-based paint can bleed through (especially if you're painting a lighter colour), your new paint, requiring you to have to paint again.
