I would lean towards getting foundations and concealers if you don't already have a wide variety. You'll need to have several in your kit to mix for all skin tones.
If you already have plenty of those, ud shadows are great. The Naked, Naked 2, and Basics palettes are smooth, super pigmented, and blend easily. I rarely by ud, but I just got them as a gift recently. I'm finding them very versatile.
I know everyone is just in love with the ud primer potion, but I'm just not that impressed with it. To be honest, the $3 e.l.f. mineral eye primer performs the same. The only difference is the e.l.f. primer is a little more opaque than the ud.
Here are some tips on building kits from two of my favorite makeup artists on YT.
http://www.youtube.com/watch?v=adALGRAEN3s
http://www.youtube.com/user/kandeejohnson/ (I couldn't fine the exact link, but if you search her channel, you should be able to find it.)
Enjoy!