You touched a subject that I encountered plenty of times, the large surface area led me away from airbrushing to first rattle cans, then to a small regular spray gun by Binks, called a "detail brush." Looks like just a smaller version of what you would see in a body shop, you're shop friend likely has one to show you.
I find I can get essentially the same results with a good quality spray can, don't laugh, I get most of mine from Wal-Mart, the cheap ones around a buck. For a large surface area, single color, it works very well for me, from primer to finish coats.
Don S. is right, opposing directions give more uniform coverage, but do remember to overlap each pass a bit, to avoid the rough texture of being too light between passes. Get some shiny, printed cardboard sheets and practice, you'll soon see how to do it, and remember to have good lighting, THAT is the spray painters best tool.
Warming the can in warm water is helpful, it gives a more fluid substance, cold paint is more viscous and doesn't flow as easily. The ability to easily blend one pass into the next is crucial. Practicing on printed, shiny cardboard will show you how close or far away to be, and I do recommend you give it a thorough try before painting the model, it can save a pricey model and prevent a poor finish.
For the camo job, by all means spray can the primer and base color, then airbrush the succeeding color patterns. You're friend with the body shop can surely give you many pointers, maybe even let you watch a paint job in progress, that would be a great lesson.
Hope it helps, best of luck.
Patrick