COMPRESSOR
One big bit of equipment you will need is a compressor. Smaller compressors are direct drive, which tend to be noisy as there is no gearing and the compressor is forced to spin at the same speed as the motor. Belt- driven machines are much quieter and longer-lived as the belt allows the compressor to run more slowly than the motor.
Size is the next major consideration, and while it is true that you can never have too much air, both cost and space considerations may mean you have to make do with slightly less than you would like. The three main figures to be interested in are the tank size, the CFM (cubic feet per minute) and the FAD (free air delivery). Manufacturers often quote CFM figures for a compressor's output. Unfortunately this is derived from a theoretical calculation of the volume swept by the piston, and does not take into account the inevitable efficiency losses within the compressor. It is, of course, more impressive than the FAD figure, which is the actual amount of air supplied by the machine. Typically this is 62-72% of the CFM figure, which is why the latter is used.
Generally speaking, a 1.5hp motor will be able to power a 6.7cfm compressor, while a 2.0hp motor will increase this to 8.4cfm. You can go all the way up to 3.5hp on single phase, capable of up to 14cfm on belt- driven models. If your setup is marginal for spraying and you can't afford a bigger compressor, rather than risk an interruption mid-spray, switching to a smaller needle, tip and air cap in the gun might just knock the air requirement below the compressor's pumping threshold and keep the air flowing.