I don't see any reason you can't use the existing system to distrubute the heat assuming it's hot water.
My preference is to have it in the house, you're going to have a lot of residual heat loss. There's other benefits as well, I'm assuming that unit comes with a hot water coil? You can heat your hot water with it too. There's digram here that explains one of the ways you can do it:http://nepacrossroads.com/download/file.php?id=1407
The hot water loops though the heater so it rarely comes on, basicallt it becomes a glorified storage tank. You can do this year round too as you will only burn about ton during the summer. Guess it depends on your situation. But we have ours in the basement and even covered with insulation it keeps it warm.
As far as the amount of coal a rough guesstimate is in the 20 ton a year range, that's assuming the house isn't insulated very well.