If you're fairly new to airbrushing, stick to single action for now. Dual acton is great down the road once you have developed your own techniques, in and outs so to speak. With dual action, one wrong tug on that trigger will ruin your perfect shell when you least expect it. I've been airbrushing over 20 years and have tried just about everything on the market. I'm now using a Pasche dual action with 3 different needle/tip sets for different jobs but I just happen to get a good deal on Ebay for it.
You have to use whats comfortable for you and you have confidence with. Start with something reasonable, not bottom of the line, but don't spend $200either. You will soon discover that most of the trials will be with air pressure and paint mixture. I personally hate water based paints, never use them in an airbrush. Well I have...that's why I hate them
Floquil thinned with paint thinner and dull cote thinned with laquer thinner. Everyone has there own formula.... 70/30, 60/40, 3 to 1, 4 to 1 etc etc. You have to determine what works best for you.
Always use a moisture trap/water filter inline somewhere
My tip....once you have spent hours on that shell detailing and prepping, mix the paint, pour it in the bottle and screwed to the airbrush......
SPRAY A SCRAP PIECE WITH A FEW STROKES BEFORE AIMING IT AT YOUR PERFECT SHELL