If your not trying to do to much detail you can probably get by with an overall coating.
Start with the face/head first then do a quick spray or brushing of the body from the neck below. It shouldn't take more than 20-30 minutes to do the whole thing. In applying makeup for the stage I learned to change characters in a few minutes.
You can also purchase a half face mask for just the upper part or just for the eyes and decorate it then do the rest of the body afterwards.
I do however understand your dilemma because unless you're familiar with stage makeup techniques it is a bit daunting for first timers.
You'll need sponges, brushes of different sizes and widths, textures. If your using latex you'll need sponges mostly and need to work fast as the latex dries and then also realize when your brushes dry they become useless.
I've had my body painted many times for different shows I was in. The first time I did my neck and arms while the makeup artists applied paint to the other areas and finally my face. i had it on all night and then went to a friends apt who helped me scrub it off. So it's best to test it on yourself by painting an arm or leg for several hours and then washing it off to see the results. Also make sure you wash any clothes separately. Lest you color all you whites to rainbows.
Latex has the advantage of being peeled off while body paint is scrubbed off.
Probably the surest way to get body paint out is go into a sauna and sweat it out.