Getting around to designing the heating system:
I would love to use radiant ceiling panels - these can both heat and cool the house, and take no floor space, and do not need the concrete floor for radiant heating. Zehnder's carboline products are expanded graphite panels with copper pipes running through. They can both heat and cool a house. I wouldn't need very many. The peak heat load is about 14W/square meter, and the peak cooling load in summer is about 10W/square meter. Delivering heat via ceiling panels is not efficient, but with heat loads so low, and the almost non-existant stack effect in a passive house, I think this might be a practical solution. In addition, it would be very easy to supplement the heat input by heating the ventilation air a little bit.
Next comes a bunch of number crunching based on:
Yearly space heat demand: about 3600kWh (240 sq m floor area, times 15kWh/sqm-annum)
Yearly DHW heat demand: about 4400kWh (based on 6 person occupancy at 25litres hot water at 60degC per person per day. Apparently this is low for NA, where the daily hot water use is more like 60litres, but typical for Europe.)