Hi Steve
Have a mess about on this website - www signprint co uk, follow the links for vehicle design, its what I used. its a bit trial and error and the only way to get a print off when your happy (unless you use that company) is to do a screen dump(print screen button), then open it and zoom in.
I personally prefer the less busy looking designs, too much information looks messy I think.
Take it round a few van grahics places see who will give you the best deal, emphasize the fact that you have done the design for them, so it's just a supply and fit job.
Paul Kinder Clean