Recently painted my Barracuda and for just doing the exterior I bought a gallon of paint. You will end up reducing it so it really is more than a gallon. The same for the clear coat if you go that route. I have paint left over and figure that is a good thing, so I can use it to do touch ups later.
I used a HVLP gun as well. Using an older design spray gun will use more paint. As far as how much, I can't answer that.
Maybe someone else will know.