I've sold Mary Kay for over two years, and definately would tell you to be very very cautious about building "inventory." Your upline, no matter how much you love her, will most likely encourage you to invest in "yourself." This means more money in her pocket.
The company usually creates new packaging for the product ever year, or comes out with some great inventory moving promotion, in new packaging.... and then the consultant is left with a lot of old product! Also they routinely discontinue products, so you are constantly buying new products and samples.
Be aware that when people say they had so much in "sales" that usually means thats how much they bought in inventory.
I am in the process of getting rid of all of my MK stuff, and I don't mean to be negative. Some people are very successful at it, and I wish you the best of luck. I would encourage anyone getting into MK to check out the following site. It's an eye opener, and kind of shows the antithesis of all the MK rhetoric.
Good luck, no matter what you decide.
http://www.thepinkingshears.com