high mass heating system

First, I think that sand is a better insulator then conductor (From "safe House" "- sand has as much insulating ability as fiberglass and walls are much more bullet proof") so I don't know if heat will "radiate" enough to offset the heat lost in the cold boiler.  In fact if I remember my camping lore "covering baked potatoes in sand by the fire will help to keep the heat in".

Second, You might be better off building a small "out house" for a tank (and boiler) with all the wood you will need to brace a floor against 52 tons of sand.  I'm glad you will be the one to "move" all that sand.

I would insulate the floor with fiberglass and run Pex plastic pipe as a radiator.  Get a batch solar water heater with it's own tank mounted horizontally so as not to interfere with you window view outside.  Tie the boiler to the tank (plumbing).  Add a DC pump so it can be powered by solar later.  Add 2 ball valves to regulate the heat going to the first and second floors.

You should have heated floors by solar (tree will not compromise solar hot water much) or by boiler.  If you do build a "power house" then consider stuffing your batteries and inverter in there when you get them.