I'd contact AHS, Axeman Anderson, EFM, for a start. I'm guessing in the ball park of 200K btu. I used 80K to heat an insulated concrete slab in my shop, but not to toasty warm temps.. I think at least 120K is needed to keep the slab warm enough to have a congregation warm... unless the sermon is a real fire and brimstone scorcher !! [sorry, I couldn't help myself
There is a lot to take into account, what is the underlying ground??? High, dry and well drained, like sand/gravel?? or is it low and damp, ocassionally soggy? Ground water will draw heat from a slab like a sponge soaks up water.. You need to put down a plastic water barrier as well as a layer of insulation under the slab, AND insulate the perimeter down to the frost line [deep in your area I bet]. This will keep the slab from loosing heat to the surrounding land.
Any of the three boiler makers above can provide 200-250 BTU boiler, most can run at lower heat settings for 2 days untended if the instalation is designed for this, a large hopper and large ashpan/ashpit. My AA260M can go 7 days on one full coal hopper, more if I wanted to raise the sides of the hopper.. the limiting factor is the ashpan... It is full after 48 hours, and will overflow if left any longer... but is doesn't cause any operational problems, I just have to sweep and vacuum up the ashes, And the full ashpan is HEAVY... I usually shovel out about 1/3 of the ash , then pick it up to dump it... Saves the back.
Once you have the heat loss/demand calculations done, and know the BTU load, then you can figure out the best boiler for the job... I'm thinking the AA260 and the EFM can have a coal hopper just as big as you want to build and load, They re both auger fed boilers. The AHS will require some special hopper setup or loading device, 'cause the hopper is gravity feed on top of the boiler.. Maybe a set of steps beside the boiler to still allow hand filling of the Hopper??
I don't know if Harman, or Keystoker make a big enough boiler for this application, the gravity feed hopper in both will requird some thought and preplanning, as well as the ashpan size. I think they are both smallish ashpans.
Hope this helps, Greg L