You could probably keep your
crawlspace above freezing with two 100W bulbs
operating all the time and a small heater with the
thermostat set to go on at 30-35 degrees.

Any recommendations for a heater like this that would be the least
dangerous to leave unnattended?

You can probably buy a complete rig designed for wellhouses,
but I still don't understand why you want to heat the whole
crawlspace when the only thing you need to keep warm is the
pipes.