Yeah, the key factor is if it currently looks terrible (like my truck did) you really can't make it worse. If nothing else, I wanted my truck to be a single color, rather than 3. I only bothered to primer bare metal spots.

Hit the whole truck with the orbital sander down to 400, basically if you can feel something with your eyes closed you'll see it through the paint.
Wipe it clean with acetone.
Super light dusting coat of paint first (basically just a light mist everywhere), and then a lightish coverage coat, and a final heavy coat to get the paint to 'wet out' so it looks like one smooth wet surface, but not to the point of runs.

I bought 12 cans of Rust-Oleum 7538838-6PK Professional High Performance Enamel Spray Paint, 15 Oz, Gloss Hunter Green, 6 Pack

I have at least a few left (maybe 3?). That came out as somewhere between matte and gloss from the perspective of single stage automotive paint (think pre 90's cars). You could probably buff it to a higher gloss, or cover it with an automotive clear (which I had always planned to do if I wanted it to be more gloss). I'm super happy with the semi-matte though, because it makes all the little imperfections really hard to see unless you go put your eyeball right on it.

Finish looks pretty much exactly the same 2 years later. When I painted it I lived in NJ and the truck got covered in quarry dust every day from blasting 1/4 mile from my house. That stuff destroys clear coats. Last June I moved to NC, and the truck has been outside the whole time since I painted it - needless to say it's holding up way better than I expected.