I'm not sure why you would want a heater of any kind for your purposes. Unless the ambient temperature of the room you will be working in is very cold, it's not necessary. In fact, more people have a problem getting their water temperature down to the proper temperature than up to it when processing b&w film - people living in hot climates, for example.
The ambient temperature of my darkroom is coincidentally almost always withing one degree of 68 degrees so I'm lucky. But I still control the temperature of my water using a Leelal mixing valve. However, for years, I got by without using one so if you are developing film infrequently, a mixing valve is more of a luxury than a necessity. A large water jacket evens out minor temperature shifts from your tap water supply. I will attach a picture of my mixing valve setup.
Another option for you that can work extremely well is possible if your room temperature is between, say, 65 and 75. If it is, you can just let your chemistry stabilize at room temperature and then compensate for the temperature using time/temperature charts.
The only time I ever heat water is when processing color film which I have just got back into doing recently. For that, I just picked up a Jobo processor but I don't use the heater in it because I found it to be much less stable than a tempered water heater that I also have. That thing keeps my chemistry to withing 1/20th of a degree F even though it is rated for plus or minus 1/4 degree accuracy. I realize that you don't have a Jobo, but for the info of anyone here that does who may be interested, what I do is hook up the "out" line of the heater to the drain valve of the Jobo and then the "in" hose goes into the main (lower) bath though one of the bottle openings. I may add an second valve to the processor to attach the "in" hose to because this works so well. My temperature control is now spot on rather than varying about plus of minus 1 1/2 degrees like it does with the Jobo heater. I'll attach a picture of my heater as well.
But, again, unless the room you process in is very cold (low 60s or lower), you have no real need for a heater that I can think of. You are much more likely to have to get the temperature down than up when processing b&w film.