Norway rats are what is most common if you're going for "typical rats." I myself have a small colony of 4 males, 10 females. Everyone else's advice has been spot on from what I've seen about cost of food/bedding.

Now I am no expert as I've just barely gotten into this myself, but you will want to research megacolon in rats. At the current time, the established colony I bought contained mostly white rats, with a few of what they refer to as "hooded" rats. The "hooded" rats (the rats that have a colored face/shoulders/front legs, and will often have a stripe down the center of their backs) are the least susceptible to megacolon (it's kinda a long story, but megacolon is linked to the color migration genetics--research it for all the info), and I'm currently holding back some of the hooded rats that my breeders have produced to make them breeders when they get big enough. Eventually I hope to feed off all of the white ones, with the exception of my huge rats since my snakes aren't large enough to take them (but like others here, I may have issues getting rid of my super-big breeders).

As for timing and breeding, you can start off with just one male and one female together. Next week, add another female, next week, add another, and so on. You can keep males in with the females and babies if need be, and typically once the female has her litter of babies weaned, she will be almost ready to drop more pinks. Now that's not to say that that's the best way to breed them. Females really should have some time off in order to re-gain bone density/general health between litters, but if you're just starting off, you may need to breed heavy at first, then cut back (though if you only have one or two ball pythons, you may not need to do that).

Hope this helps!