Don't bother, except for maybe some emergency lip gloss, mascara, and powders (a few different shades), and as someone else stated, some basic disposable application and cleanup supplies - MU sponges, clean towels, hair pins & clips, a hairbrush, curling iron and blow dryer just in case the girl forgets. First, you can't possibly stock all the items and colors a real MUA would use, and even if you did, it would get stale after a certain amount of time.
Second, there is the issue of safety - MUAs know how to clean their brushes and applicators, and to keep the product fresh so they don't pass something like an unseen infection from one person to the next. I don't even like sharing a hairbrush with someone, I can't imagine an unsterilized eyebrow tweezer. And if you did provide all those things and someone got ill, you might be liable.
Better to bring in a MUA or have the model do it herself, although it wouldn't hurt to educate a new model about the importance of using profession MU products rather than ordinary over the counter stuff. Pro makeups is made with more neutral bases (no over whitening for example) and will go on smoother.
One thing I did keep at my studio was nail polish remover. I hate red nails (unless its necessary for a particular and special shot) and so many girls show up with these awful red nails, the second thing I used to have them do is remove the polish. The first thing is to remove any tight clothing, especially bras & panties with elastic bands, to reduce the effect of lines on the body. I can't believe how many times even a pro model has shown up unprepared this way, but it happens all the time.
Andy Pearlman Studio