+1 to what missi said. A good thermostat is the best investment you will ever make towards the health and safety of your animals.

If you're going to be getting a baby ball I would suggest getting a 15qt tub to start him/her off in. They'll feel much more secure plus its 10X easier to keep the temperatures and humidity correct. A tub that size will be good for them til they get to about 4-500 grams and by that time they should be big enough that you could use that 40 gallon without having to mess around with dividers etc..... Consider looking for an adult male or something. I see them all the time on Fauna Classifieds for $5-10 plus shipping. An adult BP will be much more forgiving of beginner mistakes and you'd have the added benefit of getting a snake that is already eating well and has enough weight on them (most likely) that you would not have to worry much if at all if they don't eat for you right away.

Good luck and to BP.net!