I have a Draper low air pressure gun, works on 7-40psi so you can get away with a 25 litre compressor. I bought it in 2001 for £40 and done quite a few jobs including a spray job on a Merc Sprinter, turned out really well. But, as has been said it's all about preparation. I once bought an airless spray gun, it was total crap and I wouldn't recommend this type as you will not got continuity of an even spray pattern.