DTM Acrylic paint is all you really need. Mix with an enamel hardener and you're golden. I primed my bus right after I got it with this (in white):
It was one of the first things I did after I bought the bus because of legal reasons (in CA) and I didn't want to be harassed by the cops. This was before I really knew what I was doing, and it was a shitty paint job with an air compressor and sprayer.
Here's how it started:
After two years, the paint started flaking and the yellow came back in a lot of places. We prepped it again in the same way as before (wash, one over with a random orbital sander, another wash, then a wipe down with acetone) and rolled on the basic Rustoleum professional from Home Depot with enamel hardener.
Now it looks GREAT, even in the places where the primer had flaked off. You really don't need to put primer on, so long as you use the following two items:
As far as durability goes, I'm lucky in that my driveway is always in shade (or unlucky because of all the pine sap) so the sun doesn't damage it too much. But other than that, as long as you prep it good (with the aforementioned technique) and use the enamel hardener, you'll maximize endurance regardless of a primer coat.
I'm not a professional though so maybe don't listen to me.