You can certainly start with a 15-qt tub but even larger ones are ok as long as they are well cluttered so your snake has somewhere to hide. Make sure you get the latch box style with the handles that hold the lid down securely so your snake can't push up the top and escape.
The thermometer/hygrometer won't regulate the heat tape, you need a thermostat for that. Something like this:
https://www.amazon.com/Zilla-Reptile...qid=1467686414