That enclosure should work alright, the only problem I can really think of will be creating a temperature gradient throughout the enclosure. You are supposed to have a "cool end" and "hot end". If you look at the caresheet it lists the proper temperatures required throughout the enclosure.
Lighting of any kind is unnecessary and bulbs that put off heat will actually more often than not create more humidity issues. The heat from most bulbs pulls the moisture from the air which will lead to you having to constantly mist the cage. Ball Pythons do not need "basking spots" from above because in their natural habitat they are typically underground during daylight and come out for a few hours at night, so they do not bask in the sun. The belly heat will help aid in the snake's digestion and make it easier for it to raise its core temperature as needed. The under tank heater will be placed on one side of the tank, covering at least 1/3 of the surface but no more than 2/3. The snake needs to be able to get away from the heat if it becomes too hot.
As for your last question, the probe for the thermostat should go directly on top of the under tank heater, between it and the tank.
Hope that helps.
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