I agree with what people have said about Ben Nye making you break out. Primers probably won't help, it will just keep the makeup on your face longer, which could be worse in the long run.
Look for sensitive skin makeup, they sell them usually everywhere they sell makeup. You might want to look into powder rather than cake foundation, it'll be lighter on your skin.
From your post, it sounds like you feel you need to make your skin darker for the cosplay. You really don't have to! You'll look fine without it! Using makeup to look darker will make your skin look oranger rather than tanner. Here's a pic from Deviant Art of a lighter skinned Korra, she's one of many and she looks fine: http://browse.deviantart.com/art/Korra-312647133
For makeup advice, try searching "oily skin makeup" or "acne prone skin makeup" on Youtube, you should be able to find some makeup brand recommendations and tutorials. I would suggest then practicing on your own and finding the look that seems the most "Korra-like" on you! Since you say you're new to makeup, I'd recommend even going into a department store and getting your makeup done. Ask them for a "natural look", tell them your skin is acne prone, and ask them to narrate what they're doing. It'll help you learn about application (just as important as which products you buy), they can recommend what products to buy (don't feel obligated to buy them if you don't like them/they are too expensive), and if you like what they do you'll have a basis for what your makeup for Korra could look like.
I'm actually planning on cosplaying Korra later this month, and although I don't recommend trying to look tanner, I do have the problem of a very uneven skin tone (I have a bit of a T-shirt tan, so my shoulders are very pale while my forearms are really ran). So if you're interested in something for your arms, I'm looking into self tanners and bronzing lotions. I haven't tried it yet, so I can't say if they work, but as long as they are used a few days before you wear the costume they shouldn't rub off on clothing.