What is Reddit's opinion of Grundfos UP10-16BN5/LC 96433897 1/25 Horsepower Circulating Pump with Line Cord?

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Use a propylene glycol solution with an inhibitor package added. Conventional automotive antifreeze will probably work great. Dilute it to provide freeze protection for the coldest ambient conditions you would ever expect for your geographic location. But higher-concentration does not equal better so don't over do it. Specific heat of p. glycol drops off quickly with increased concentration.

You will need to have an expansion tank to give the fluid some place to go as it expands and contracts assuming that you use a closed system. You should also consider an automatic air vent as this will make getting the air out of the system much easier than trying to do it manually.

Shoot for 3-7gpm in each 3/4" circuit. Without knowing the type of pipe, number of fittings, etc, it is not possible to exactly specify the pump but a 1/25HP water circulation pump will probably do just fine.

I'm not familiar with residential control systems for hydronic heating but on the industrial side we would use a temperature probe at the discharge of the heater and modulate or cycle the heat source based on this. I'm guessing there are probably integrated controllers for residential that incorporates both pump control and heater control together. You might check with /r/HVAC. You might also pay a visit to your local HVAC supply house.