I built a film processor a while back. It used an Auber temp controller, platinum rtd , 300w titanium aquarium heater and a fountain pump to circulate the water in the bath. Once up to temp it varies maybe 0.2 degrees C. I used standard SS drums(either 4 roll or 2 roll) with small bolts soldered on the bottom,which hook into the drive so I could pull it in and out and change solutions.The tank itself runs on a set of plastic rollers. For the drive I used a automobile window motor, and an electro-mechanical system to turn the tank back and forth. This seemed a lot simpler to me than microprocessors and such .
I've run about 70-80 rolls thru the thing, and the only hassles are taping up the top of the tank to make sure it doesn't leak, and a sticky relay. As for keeping the solutions warm I used a wide enough tank(plastic storage box) to let me put 4 soda bottles in it. That covers 1st and color developers and I add a bottle as one is used. also don't over-insulate the tank. I did that and the temp controller couldn't hold a temp. Once I ripped off the foamboard , it worked great

Now I want to try out a version for 70mm film