1. One that hasn't been neutered. They aren't the best reproducers if they don't have their junk intact. Otherwise, any kind. Rats are prolific. Just go with one you like.
2. Housing a pair of females together is fine. They will help each other care for the young and keep each other company. I like to breed my females a few weeks apart that are being housed together though. That way you have less issues with a female baby hogging.
3. Some people run harems, not removing the males at all, but I disagree with this method (for the most part). You aren't giving your female a break in between pregnancies and she won't have as many offspring and will die much younger. Just place the male in with the female you want to breed for one week or until you notice her being pregnant. Then put him in his own enclosure, preferably with another male so he doesn't get lonely.
Also, to add something. If you are looking to save money, you will be unsuccessful only breeding for two snakes. If you are aware of that fact, it's another fun aspect of keeping snakes that many enjoy.