Roy-
I'll toss in my two cents too. In addition to what everyone else has said, how good the paint looks after it's dry is about 80% prep, 10% paint quality, and 10% technique. It doesn't matter if you use $100/quart PPG Delstar and have been painting for 30 years, painting over crap is going to look like crap. I wipe my parts with a rag soaked in mineral spirits immediately before painting to remove any dust, wax, oily residue, etc.
Thin the paint by the manufacturer's instructions if there are any. Sometimes you have to modify them slightly to get things to work. For example, I've sprayed white that I could only add hardener but no reducer to, or it would run off the parts...
Having said that, actually spraying the paint is about 75% technique and 25% paint gun quality in my opinion. Perhaps you've seen pictures of the 100 I redid last summer...I painted that with this old POS Sears paint gun. If it's dry tomorrow I'm going to try to at least prime the rearend and some other random stuff for my 1872 with a cheap HVLP gun from Harbor Freight. My dad used it to paint fenders for his trailer last year, and they turned out good, so now I need to practice with it a bit, too. Basically what I'm trying to say is, you don't need a great, or even good gun for a nice paint job.