I'm definitely nthing the idea to apply for pro-memberships thorugh various companies as the way to go.
Personally, I really do NOT like Mary Kay makeup. I love their body stuff, but I personally think their makeup is horrible. I've had major issues in both wear and pigmentation throughout the whole line. Now granted, I haven't tried any of their stuff recently, so they may have change their formulas. I don't know, and I'm not keen on finding out, lol.
Also, another thing to think about is this: if I had hired you to do my makeup, and you pulled out a bunch of branded Mary Kay products, I would be furious. Why? Mary Kay is primarily sold through makeup parties, where the seller normaly comes in and does your makeup using the products as a demo.
For free. I realize that your skills as a MUA are going to be greater than the average MK rep, and that is what I'd actually be paying for, but you're not going to find that acknowledgement from everyone. Bad word of mouth will really hurt your fledgeling business.
I agree with studiomakeup in that you can build up your storage for less money with some inexpensive product lines. I love Rimmel's lip colors; their formula is my favorite.
Cost does not equal quality, and that's hard to drum into some people's heads, lol. My favorite mineral foundation right now is actually Wet 'n' Wild's, and I think NYC has a nice foundation for $3.