A word about Photoshop itself: it is the most powerful editing program but it is very very difficult to learn to use. I own and moderate some photography lists where photoshop is a major topic. And I tell people to expect to spend 3 to 4 months working a couple of hours per night (with a good intro book to photoshop) to get to where you have a basic understanding of how to use the program. And that doesn't really cover the the graphic tools that photographers rarely use.
I'm sure there are a lot of custom paint shops that could convert your hand drawn images to a graphic file. The suggestion to get some desaturated images of the truck-both sides, front and back (top would be good if you can find somewhere you can shoot down at the truck) And print a number of the views to let you draw on them is good.
BK