Here's what I would do:

1. Purchase a live rat fuzzy and have the breeder/owner kill it for you and you put it in the freezer. Then thaw it out and see if the snake will eat it. There's no point buying a bunch of rats if your snake won't eat them. I bought a bunch of rats/mice for all my snakes (I have 9) but I have two who only eat live. So you want to switch them to frozen/thawed before you bulk order.

2. As far as bulk ordering goes, I would buy a small bag (no more than 25 per bag) of what they are on, and then one bag of the next size up, a bag of the next size up from that, and so forth and so on as long as your budget permits. If she's eating frozen fuzzy rats, I'd get a bag of fuzzy rats, bag of rat pups, and a bag of weanling rats. Then when your snake out grows those, then buy a bag of smalls and then several bags of mediums because that's what adults will eat.