Hello Coalboy, welcome to the forum.. The most accurate way to answer your question is to do a heat-loss calculation..
But with an older house, you may not know how much insulation is in the walls or in some hidden ceilings [my problem in my 1849 farmhouse].
So another way to figure it out, is to look at your current heating system, what is it, how old is it, how well does it work, and how much fuel does it burn.
You can interpret BTU used from the amount of fuel burnt.
You can interpret BTU needed by the BTU capacity of the current system
For example, my house has a 140,000BTU input, 120,000 output propane boiler.. once the outside temps drop below ~15* it cannot keep up with the heat loss of the house and some parts of the house cannot maintain 55*. So obviously my home needs more heating capacity [and insulation/sealing/windows etc].
So tell us what you are using for fuel, and we can direct you to a fuel calculator, and describe the type of heating appliance or system, You mentioned boiler, is it steam or hot water? Cast Iron radiators, baseboard units or Hydronicly heated floors?.. Any added Electric baseboard heat.. or ???
If you are going to keep the existing system as a backup system, do you have a second chimney available??
As for your list of boilers, there are some fundemental differences.. the EFM's use a separate coal hopper with an auger to feed the boiler.. this hopper can be a large as is practical for your instalation.. my hopper holds 1500# of coal [AA260 boiler].. The Keystoker and Harman have a hopper that is attached to the boiler, and is a set size.. and usually needs filling every day or two during cold weather.
So you may want to think about how often you want to tend to the boiler, the hopper size and Ashpan size will determine this, as well as the amount of heat used..
Hope the above will help us help you with your questions..
Take a look at his topic, there are a couple of links to fuel calculator/conversion calculators listed in the discussion. please list your annual coal usage VS. oil usage