You need to decide on a finish product type that you will be using first. Some products such as polyester require a large needle size ( 1.8 to 2.5) compared to lacquers which spray stock tips (1.5 or so). The wrong tip size will result in poor finish no matter what gun brand or price you pay. Having to replace the tip with the right size can cost as much or more than the initial price of the gun. Settle on a product, get the specs. for the gun needed then buy. Personally, I have had best results from HVLP gravity fed guns. Cheap ones seem to work about as well as the more expensive stuff and up to 95% of the quality of your result will be in your technique.
Best,
NV