When starting my own professional kit, I referred to Bobbi Brown's Makeup Manual religiously because in the book she goes over the necessities pretty thoroughly.
You need at least 15 shades of foundations, and in other formulas considering skin type, desired texture, etc. A valuable tip I learned from her book is that, considering the amount of people you will be working on, your foundations will NOT match them all. When it comes to any photography, foundation must be flawless, so if you need to match an unruly skin color, add yellow, black, red, or blue pigments to the formula.
Other things I would have not thought of without the lovely Ms. Brown:
Baby scissors for eyebrow hairs (since you can never have expectations for your models/clients)
Eyedrops
Self Tanner
5 shades of tinted moisturizer
Brow gel
Baby oil
Lip balm
An extra brush set
Mints (could be you or the model in need of freshening)
Spray-on brush cleaner (for immediate sanitation)
I just want to reiterate what everyone else was saying with sanitation because it is just as important, if not more so, as the finished look itself. Do NOT keep products past their expiration date, such as mascaras, (even when using disposable wands). I would seriously stray away from any roller eyeliners, as opposed to pencil, unless you want to waste a lot of product sanitizing.
I hope this helps a bit more as well, lovely.