I have 2 guns made by CA-Technologies in Colorado. They are $300-500, depending on the configuration (3M PPS vs Aluminum cup, number of needles). Although the sets that I have bought are for automotive, I use them for applying wood finishes (shellac, lacquer, waterborne).
I would recommend a gun that has service parts available (DeVilbiss and SATA are used widely in auto industry). Stay away from cheap guns.
Then you need to decide HVLP, RP (also known as High Efficiency), LVLP. These have different application speeds, CFM consumptions, and quality of finish.
Lastly, practice a lot before you are satisfied with the results and decide to paint your truck.
Good luck!