Hi again!
Ok, let me just briefly explain what it is about before getting involved (both of us) in something that is out of question. I am not sure if it is a appropriate system for several reservoirs with that much space in between.
I've once build (long time ago) a floatation tank that held nearly 1000 Liter (some 250 gallons) of water that needed to be heated and kept constantly at 34.5°C in a rather cool environement (indoors though). I did so with a small 2Kw water heater (we call that "under sink water boiler") and a few meters of garden hose, a small water
pump, plus some plumbing fittings and simple house hold stuff.
If it happened that you already have a used 2-3 Kw electric water heater (because these aren't exactly cheap), it's done in a day. Closed hot water circuit but not under pressure. No supplementary electronics needed, if the heater has a simple but "continuously variable" temp regulation. I am not sure about the terminology here but I bet you know what I am talking about.
So please, tell me if such a solution is of interest for you and if you got the stuff or are willing to get it in case...
Cheers,
Luches