Do not know the temps you plan to work with but alcohol should be cheap realatively but may make the mixture boil at too low a temp. Sorry but denatured ethanol is as close as I can see to nontoxic. Do not use straight methanol it is very bad and is often used to denature ehtanol. Your other cheap way is salt but it is going to play H--l with the rest of your setup other than plastic. Look at what they use for deicing compounds such as calcium chloride or magnesium chloride. Not sure what metals you will use in your setup but that will have a big bearing on what can be used for antifreeze.

As I remember he had placed tubing on the floor, the same way You are thinking, but he discovered that the heating at those latitudes you need excessive floor insulation -- I do not remember what he had -- Reason why I am looking for him, because I am assisting another fellow with the same problem and searching for a practical solution.

He had the tanks coupled to a car radiator with a fan to circulate the air and a small pump to move the hot water to a colder water tank -- I think the was able to keep the house in the 70s F without much use of the propane heater.

About your eddycurrent heater.
Sure you need a metal plate behind coppertubes.
If this plate is stationary it will 'absorb' much of the energie
of the moving magnets,so in this case it would begood to have the tubes
welded/soldered to this plate to absorb the heat.
If this plate is rotary with the magnets or laminated,it will provide a returnpath
for the flux but won't absorb energy.In this case I don't know
how efficient the copper tubing stator would be.Probably it depends
on the amount of copper and maybe on other designfeatures of the stator.

I think you can use regular old H2O if you put in a drainback valve, which drains the water out of the system into a holding tank when temps get below freezing. Water tranfers heat better than antifreeze as well.

Volvo Farmer

Assuming this is a closed-circuit (sealed) home-brew solar-heat-transfer set up, what about veggie or mineral oil?

Veggie oil would be cheaper,but I'm not sure about decay issues. (Would it decay?....I've had bottles of the stuff on the shelf for well over two years, and never noticed any changes..)

Mineral oil on the other hand is still petroleum based, but if it's Bio-safe enough to administer internally for constipation, and to rub onto a babies skin (baby-oil), then I doubt you'd have much of an environmental catastrophy should you spring a leak.

Both would be freeze resistant, as well as tranferring their heat as good as any liquid, I would think.

Hope this helps
Bruce S