Amen to that!!!
In my opinion, there is no way to have a serious coal central heating system without a chimney.
As far as what boiler; keep in mind that the AA or AHS 260 has a lot more output than the EFM 520. I think you would be in EFM 700 territory if you want to beat a 260.
Again, I don't think there is any problem with your current boiler. If you put a bigger stoker underneath it, you would have all the heat you need. But, that's a whole conversion thing that you might not want to get involved in.
Chimney first, then you can dream of what coal burner to get.