If I could only have 2 guns, with a limited budget, I would choose a gravity feed HVLP full size and a gravity feed jamb gun for small areas. First of all HVLP requires less air volume at the inlet and gravity guns leave little waste. Both of these are major considerations for the small hobby shop. They would also be much friendlier to the environment and the HVLP system requires 30 - 50 % less paint. Personally , I would never mix paint directly in the container/jar. All paint products have major contamination. Thus your paint should be strained through filter papers into the jar. Mixing directly in the siphon style container eliminates this precaution. When you buy your paint, talk the counter man into supplying you with "FREE" mixing cups, sticks and paper strainers. Many of them will. I have yet to use the Harbor Freight guns, but just might get one of their gravity feed HVLP jamb guns.
P.S. You don't want to shoot build primer with an HVLP unless maybe you get a fat tip. Actually you probably don't want to shoot primer and finish paint with the same gun.