The vivarium size is going to depend on the size of the snake and what type of enclosure you're going with. For an adult female, plan on needing a 40 gal glass enclosure (which I don't recommend for various reasons), a 4ft x 2ft pvc enclosure, or a 41qt plastic tote (my favorite and recommend type of enclosure). You won't ever need anything larger than that.
If you're buying the snake as a hatchling. Deborah has a great guide on how to set up a 6qt tub for it.
I recommend a Spyder Robotics Herpstat. They're very reliable.
As for health... buy from a well known reputable breeder. Buying from Craigslist or a chain like petco is one way to guarantee your snake will NOT be healthy. Good breeders can be a bit more pricey but IT IS WORTH IT.
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