Avoid the tankless. They cost a mega ton to buy and install, the require frequent repairs and repairmen are hard to find, and the operating costs are huge. I think the best course of action would be the combination coal furnace for the winter and an electric heater for summer if the transition between them could be handled OK. Since the coal fired furnace is being used for heating the house, it costs nothing extra to heat the water in the winter. The pros may well have some additional (better) ideas.