i like iwita in a double action it will give you more control if you are doing fine work like around eyes ect. gravity feed is the best i think. you can use low pressure an it is great for shade work an fine line.for large scale go with siphon feed it takes more presure but will cut down on work time an control paint flow better. air brushes are internal or external mix internal will most often produce a nice finish the key to paint being sprayed is it being atomized here the key is kind of paint, mix, retarders or thinners, air pressure, an Ambient temperature nozzles an needles also play a part
i don't paint any game heads i am a sign painter but these other guys like nctaxi can help you more on that end i hoped this has helped you some
PS practice practice practice an buy a good gun good luck !