The cheap HVLP guns I've used were $30 on ebay, 1Liter gravity feed cup ( 33 oz ) and come with 1.4 1.7 and 2.0 mm tips.
I prefer the bigger cup simply for more capacity, but the taller cup may get in the way if you're painting underneath and inside things.
I purchased one originally as a cheap replacement for a broken HVLP turbine system I was using strictly for primer, but for $30 it actually sprayed just as good as guns I've used costing over $500, and it has held up pretty well for a good 6-8 years now.
I prefer the gravity feed because they typically can handle higher viscosity primers and single stage paints than a siphon feed can, and the 3 tips allow better volume control for everything from thin basecoats , medium viscosity clears and single stage to heavy primers.
Also, the last siphon feed HVLP gun I had used a pressurized cup and any leaks around the cup seal ( due to my laziness cleaning my primer gun thoroughly ) would cause difficulty holding pressure, which caused obvious material flow problems.