The best choice is an automatic-reservoir compressor, which does not run constantly but maintains the constant, steady pressure required. Remember, many expert painters suggest that moisture traps and a gauged, pressure-regulating valve are worthwhile accessories for a beginner's airbrush unit. Build a simple paint booth with an exhaust fan. Keep a paint respirator handy and use it faithfully. Be sure the paint you are using is appropriate for the material used in the model's construction.
Some plastics require special paint.
Check to see whether a primer is necessary. Practice with your new airbrush. Use scrap metal and plastic as target materials. Remember, the key to getting an even finish is to start the airbrush to one side of the target area, spray with an even stroke across it and go past the area before releasing the trigger. Practice, using scrap materials, other applications such as thin lines and shading from one color to another.