I was in a similar boat recently but with different requirements. I didn't already have coal and need an all around heat source. Looks like you already have a coal boiler...
I went with the Hitzer 503 insert. It does need electricity to operate most efficiently however it will work without it. I've tried it.
I looked at other options including freestanding units. My hearth in my dining area is short and I couldn't expand it easily to retro fit a freestander so I went with the insert. Fantastic choice! This thing is just amazing, I love it and it keeps my house between 78-68 depending on the room. This is a 2400 sq ft, 200 year old home.
That said, for you application (and having gone through a similar thought process) I'd recommend a freestanding stove with a hearth extension. Of those that I looked at I was impressed with the Hitzer's (30 and 50 radiant models) and the DS Machine. Both offered outstanding value and required no electricity. If you are looking for something a little more dressy (and apply priced) then Harman is good too.
Carrying two different coal types would be my opposition but you may have the room.
Good luck and I'd be happy to answer any questions about thought process, stove and install. I did everything myself.