I have used a sharpe SFG gravity feed gun (HVLP) on a 5hp with I think a 20 gallon tank and it worked ok, however, I was shooting motorcycle bits so it was like the gentleman said - small panels. I didn't run the gun hard because you are constantly moving about trying to get the paint on the tank which has a lot of curves and keep the paint laying down 'absorbing' the over spray.
The biggest paint job I am on now is the Great Rust mountain, but I have taken it apart and am now shooting it with a 2 qt pressure feed remote tank chinese schlock-up gun, but, again, I have pulled the body work apart and am just shooting fenders and hood and other bits. I also am shooting them flat (one side at a time laying on a table) and I am using Zero Rust for everything as the truck will be 'safety red' with some clear coat - white accents, and black frame bits. I also now have a 7 hp 60 gal compressor -but mainly got that for the sand blaster.
With the parts laying flat - you can really dump on the paint and you don't have to be such an artist. Also this is not going to be a 100 point con-coors winner - just a good old truck that's earned its dents. I am also painting outside so, its sort of like school food - what was that figure for acceptable number of rodent hairs and insect parts?
The trick now is to lay on the paint, make the truck look wet, and then keep going so as you paint the overspray doesn't 'cloud' the paint. However, if you put time and effort into it, you can fix any sag and sand/buff out the overspray and put clear on it.
Prep is 90% of the job - nothing majicq happens when paint goes on. If the base is smooth and dust free...
I have found the acrylic enamel to be the easiest to shoot paint I have ever used. Which includes various urethanes, etc. Zero rust thined is a bit on the thick side and works best in a pressure pot. Clear can be hard as it needs to be put on a bit lighter than the base coats. Shoot a misty tack coat of whatever paint you are working with, then you can put the good coats on after that. The tack coat helps hold the paint.
hmpf...that got long winded...oh well.
I think you might get away with it, as the 5 hp compressor is just at the bottom of what you can use with an HVLP. I also have pressure guns, but don't use them anymore, after I got used to the HVLP, which kind of spits the paint onto the surfaces, however, much less overspray and fumes and mess. Paint seems to be formulated to lay down more with the HVLP so it works really good on pressure guns.
There, that's enough ignorance that will probably have the real painters tellin me what a dope I am.