Hi Jason.
I think, because this forum is based in the USA, that most folks don't know that a lorry is a truck.
You could invest in an auto outline CD, that has scaled drawings of trucks, for starters. Or simply take a photo of the truck you plan to letter with a dollar bill (I know you don't have them there, but whatever paper money you have that is 6" long) taped to the truck to help scale it up in your sign program, or just measure with a tape measure.
If you're new to this, maybe practice on your own vehicle to get a feel for the curves.
I'm assuming you are doing these in cut vinyl, not signwriting.
For designs, get yourself a copy of Mike Steven's
Mastering Layout for a good start on the best way to design vehicle graphics. Or look around to see what appeals to you and try to emulate it. remember to keep it simple and never stretch text to fit.
Remember to never do anything without a deposit, and to get the average cost of this service to avoid being a price cutter and having your local sign guys hate you.
Love....jill