It should stay above 15C most of every year. I would only let it dip below that in Fall as part of the preparation for brumation.
If night temps are dropping below 15C, then you don't want a heat bulb. It needs to be dark in the enclosure. You will need to get a ceramic heating element (CHE) or a radiant heat panel (RHP) and a thermostat to run it. Either of these heating devices will emit heat, but no light. The thermostat should only cost around $20-30 dollars, and it will keep the temperature near where you set it.
Yep as Tom has said, you’ll want to get a CHE(ceramic heat emitter) it’s a non light emitting bulb used for night heat. You’ll want run it on a thermostat. Never go with the red night bulbs they sell, they aren’t appropriate.