1) It is not necessary to keep them in a larger enclosure, but if you did, don't make it too much bigger as they begin to get stressed in very large enclosures as they are used to very tight hiding spots in nature. You'll have to do the math for dimensions, but they are only 6-8 inches high.
2)The heat should only cover 1/3 of the bottom. They need to be able to get off the heat if they want. The difference between them and geckos is not what ratio of the bin has the heater underneath, but the size of the bin.
3)You'll have to just do your research for the area you live in. Personally, I really like normals. Spiders, pastels and yellowbellys are usually on the cheaper side although if you are worried about having enough money, make sure that you are prepared to pay hundreds of dollars in vet bills at any moment. You don't want your finances threatening the lives of your snakes.
Good luck.
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