It's all kinda relative, but if you are going to use Rustoleum for $$ reasons, I'd just rattle can it with FLAT paint. If you have a desire for anything glossy or even kinda nice, or if you're painting more than one car, use the gun and spend a few more dollars on catalyzed paint.
Also, gloss black is also the hardest color to paint nicely. It shows every flaw. But the fiberglass top is textured anyway so that's probably easy to do with 8-10 rattle cans, even if you use the glossy stuff.
I don't know about how much paint you'll need, but I used about 1 gallon to paint a 69 VW beetle. You might need more for a truck. If you use spray cans, plan on getting a few cases of them.

I'd expect to spend an entire day removing parts you don't want painted and taping off everything else you can't remove.
Painting is kinda the easiest part (compared to the prep work), the prep is really time consuming and easy to screw it up.