You can use almost anything to heat it, even a campfire under the barrel. Temperature is not so critical just make sure its between 120-190. Cleaning power is not so good when temp is low.
I used electric hot water heater. Power is 2000w. It takes 4-6 hours to take the temp up but then its hot very long time before it uses the heater again.
Any thermostat or so will work. I have heat sensor in little pipe bottom of the barrel and that digital box cuts away power if temperature is getting too high or if its getting cold it takes the heater on. My friend brought it to me and connected the wires so i am not sure how it really works.