[Fiddlebog]

Woody is right about the prep work. I use a cheap gun, and every time I've had bad results it's been due to me not properly prepping a surface.
Your best place to spend money here is going to be on a good quality, wide variety of sandpaper as well as good products such as primer (I only use SEM) and a good variety of filler products (Bondo)
With the gun itself, the biggest factor is going to be having the settings right.