As for the paint, you'll probably want 2 things-- one for your face, and another for the rest of your body.
For your face, you'll want a theater-quality paint that will stay on. Mehron's paradise line (which is what I have on in my icon) works really well even in heat/humidity. Ben Nye is also a solid choice.
PAX is the way to go when it comes to painting the rest of your body, though. Note that body painting like this will mean it'll take you a few hours (yes, hours!) to get your whole cosplay on.
Personally, I'd invest in a bodysuit of sorts so that you could just paint your face/hands. A bodysuit would also give you something to attach Stitch's extra arms/spikes to, if you were going for that.
Good luck, and can't wait to see it!
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- Rose Lalonde "Glow" Tier (Homestuck): 95% (Detailing) Otakon 2014
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