I'd also look into a reptile rescue group to adopt a large snake like a large boa or a burm, that way you won't have to wait for it to grow up. I've adopted all of my burms from my herp society, we're always looking for people to place big homeless snakes with. I'd also start out with a larger group of rats. You never know if they're all going to be good breeders or not and if you do produce more then you need, you can always freeze them at the right size. They don't really breed like clockwork and you'll have times during the year where you have too many, and times when you won't have enough. Even with the same group of breeders you'll have peaks and valleys of production throughout the year.

Mark