No I understand completely!
Sculpted customs are usually clay sculpted directly on to the plastic pony. It sounds to me like you are seeking more of a mural or painted only custom. ^^ They tend to be cheaper anyway so that's a plus!
Airbrush is good for gradients, dye is up for debate about whether it works well and holds, and painting is the standard but you'll want to get an artist with good technique to avoid brush strokes.
For the hair it's largely about if you want it curled/styled/etc. Nylon is the standard and many people use dollyhair.com or retrodolls. There is lots of color choice.