IâÂÂd say the short answer is a boiler for hydronic in-floor radiant and mini-split HP for A/C & heating redundancy. Perhaps some ceiling fans as well.

SR

I'm a homeowner, not an expert, but doesn't an electric boiler rely on resistance heating? I don't care how efficient a hydronic system is, I think that will be very expensive to operate. Your HVAC contractor should be able to help you assess how to balance initial costs with operating costs.

An alternative to consider would be an air-source heat pump, the line designed to do better than competitors in lower and sub-freezing temps. (One of the regular experts will supply the name). Along with a high-eff propane furnace as backup for the extreme temps. I'd put the ducts on the inside-side of the walls in bulkheads (also called "inside the envelope") to improve efficiency and reduce heat/cooling loss to the outside.

Since this is a new building, be sure it's tight, has plenty of insulation, and that the steel structure has thermal breaks so that heat and cold aren't conducted inside by the skeleton.

I won't do the calculations for heating with propane vs electric. If you are going to heat with electricity why heat water first? Can't the electric heat be buried in the slab? If for some reason some other fuel source becomes less expensive, install forced air using that.

On the other hand, a heat pump producing hot water could serve you better in the long run for radiant heat in the slab. I am not an expert, but my impression is that is more a commercial-type installation. I know that Mitsubishi City-Multi systems and similar from other manufacturers will make hot water, but they may be too big for your building. There are probably other options, but they might be outside the experience of a strictly residential installer.

Carefully consider building orientation and window location for passive solar heat in winter. If you have good insulation and other efficient features, a high efficiency heating or cooling system is less important and you will have to invest in smaller mechanical systems.