I had a Harman Magnum, and was very pleased with it. My existing propane boiler was failing so I was in need of a replacement for that unit. I researched coal boilers on this forum and online, and ultimately went with the Harman VF3000.
Here are a couple of links to threads comparing the various boilers.whats everybodys opinion on keystoker boilersKeystoker or Harman Boiler
Size wise the Harman is about the same as a Keystoker KA4, smaller capacity wise then the AA/AHS 130 boilers, the KA6 and the EFM520. Keystoker no longer builds the KA4, only the KAA2 and the KA6 are now made.
Keystoker, Harman and EFM produce an oil burner option for their units. Harman gets about $600 for the oil gun option. Its a Wayne Blue Angel burner that mounts in place of the main access door. You still have acces to the grates, even with the gun mounted to the new door. I'm not sure how Keystoker and EFM mount their oil guns but I think they are fixed somewhere on the boiler.
I think that the auxiliary oil option is worth considering, as opposed to having two boilers especially if you don't already have an existing oil burner. You can't share a single flue with two boilers or furnaces, and if you decide to sell your house in the future, prospective buyers will look and see an oil fired boiler, something they are used to. A single boiler will take up less space, and these coal boilers have large heat exchange surfacea areas, so efficiency should be pretty good. Harman claims 84 to 85% efficiency with a 300 degree stack temp for their unit. The Harman is a two piece boiler, and can be configured with the flue pipe on the left or right, the Keystokers have to be ordered from the factory in the configuration you want.
All the boilers listed are good units, the AA, AHS and EFM will set you back more than a Keystoker or Harman, you have to decide if its in your budget or not. Try to see the units your interested in, in person, and talk to people that have one to get their opinions.
Im on my first season with the Harman boiler, it is working well, it keeps up when the temps have dropped into the single digits. Im saving at least $2000 in propane this winter, so the payback should be in 2 to 3 years. In the summer months, it makes my hot water for me, and burns about 12# per day. I'm happy with my unit, and would not change my decision.