The Following are tools for the job:
This may be obvious, but I've put it in anyway. Brush painting a helmet is a none starter, a quality finish is all important to get that stand-out from the crowd appearance.
Constructing a compressor:
If you have one, or have access to one your half way there. If you don't, all is not lost, my first was a fabrication of recovered bits'n'bobs. You will need a compressor from a fridge unit complete with the electric cable. Some form of receiver, I used an old 9ltr water extinguisher as these are pressure tested to about 250b. Also consider an air reservoir from an old wagon.
Connect the outlet of the fridge compressor to the receiver with air hose, attach a regulator to the receiver, this will provide control over the outlet pressure to your airbrush. Some form of pressure trip device is required to turn the compressor on and off to prevent exceeding the working pressure of the receiver. And basically your away, my costs were around £30 to construct it as most bits were from the scrappy.
Airbrush
Basically all you need is any air brush with two stage control. This means you can control both the air and the paint, operating is air on -paint on-paint off - air off. in that sequence. Most cheap air brushes are press once and get paint and air together, these are difficult to control and are classed as single stage. Spraying pressure should not exceed 40psi / 2.8b.
Air-Guns
These are required for priming, lacquer, and large areas of colour. The wider the fan of spray the better for a more even coat you may also need a special needle for the lacquer, but it's possible to get away without one.
Stand for helmet
I use a 3"x 3" plastic tube about 22" tall, fastened at the base to an old record turntable, this allows the helmet to be placed on the tube and rotated as required. Which avoids the need to touch the helmet once painting starts, as the grease from grubby fingers can make more mess than you think. I use a longer stand when lacquering to avoid raising any dust.