If you do a hydronic (wasserheizung) system, use a heat exchanger and a small pump to separate the heat source system from the floor heat system. It will be easier to control the temperature instead of having continuous 100°C+ engine coolant flowing through the floor.
I'd suggest using PEX tubing for radiant floor heating. It has an air barrier wall to prevent oxygen from entering the fluid system which could cause corrosion. Using a product like Schluter Bekotec glued to the metal floor will provide insulation below the heating pipes, and then fill the voids with lightweight concrete to help transfer the heat.