1:has anyone done this before? Yes loads, myself included, but outside of restorations where the originals were stenciled, no artwork painted by hand (excluding airbrushed work) has been particularily memorable (Except mine!

).
2:are there special paints i need to use? Depends on the surface your painting. Acrylics work well if you can paint them smoothly. an airbrush is best with an enamel based paint.
3:will it end up costing me more in the long run?Oh yes, especially after you spend a fortune on paint, equiptment and clear coat to protect it.
4:will it be as durable? 
Just remembering the day I moved house and put a big scratch through the handpainted artwork. Had I originally sucked it up and paid the print costs (I thought they were pricey at the time too!) I could have just replaced it rather than having to strip down and re laminate one entire side of the cab!
Anyway just my £0.02