Hello George, and welcome to the forum. I personally would recommend a coal boiler and use a water-to-air heat exchanger to put the heat into the furnace duct system.
This provides at least two advantages. One, much more controlable heat, with the ability to absorb extra heat in the water as the coal fire backs down [when the house thermostat is satisfied].
Second, a built-in hot water source for domestic hot water. Do I hear unlimited hot water, 45 minute showers and laundry at the same time??
The system you described is usually called a heat-dump system, and it is in place in most boilers, and big furnaces. You have to be able to shed the excess heat when the coal fire is trying to make max heat, and then suddenly the demand for heat is shut off [thermostat satisfied]. The coal fire can't just turn off like oil or gas.
In a hot water system the water can continue to absorb the heat from say 150* water all the way up to 220* water, That 's a lot of BTU's. In a furnace, the fan will have to keep running until the temp of the heat exchanger drops to a safe level to prevent damage from overheating. This means the the house will get way too hot just when you don't want it to get warmer.
Here is a thread with lots of links to boilers and heat exchangers:
The US Stove units are wood/coal units right?? I would shy away from these, unless you want to be able to burn wood too. The compromises for burning wood often greatly affect the quality of the coal-burning experience. I made my boiler to burn both, I REALLY wish I had built it to burn just coal. I've had to learn and modify a lot to make up for the compromises made for wood-burning.
There are several high quality coal units that can burn wood, the emphasis on burning coal very well. Not visa-versa.
Hope this helps. Greg L