I agree with Hickly. First aid supplies and fly spray are crucial. If she's going to be in a box stall at all, make sure you have enough bedding. Also, you can never have enough brushes, curry combs, etc. I'd also have a few extra halters and lead ropes on hand, as well as the usual assortment of tack.
I know this is fairly generic, but I hope it helps. Good luck!