Up until now I have tried to help the original poster correctly evaluate the load, but the focus of the discussion seems to have drifted.
It doesn't really matter which boiler casts a bigger shadow, you need one big enough to heat the houses on a very cold day, have some additional capacity to overcome piping losses, and finally...I would allow some capacity to help you in the event of poor quality coal, or a boiler that is overdue for a cleaning when that cold front rolls in (I shamelessly stole that last line from forum member Pacowy).
To the original poster - You have determined that the installed radiation can keep the houses warm, and you have measured it, good work. You have observed a low temperature drop over the pex runs, and that is also good. If you have a means of supplementing the heat to one or two houses in extreme weather, that might tip the scales one way or another. Boilers only come in so many sizes anyway, but a unit with an adjustable feed rate is more forgiving in operation over a wide range of conditions.
My opinion isn't even worth $0.02, but I'll give it to you anyway. 340 feet of radiation, 550 btu's per foot, 15% piping and pickup, and 20% on top of that for a "bad day" = 258k btu's per hour. You'd be hard pressed to move more than 210k through three sets of 1" pex lines anyway, so any boiler in the 250k ballpark should be reasonable. EFM 700 or 900, Keystoker K12, AA260...something like those. I fully admit that I am a big fan of EFM products...I have installed three 520's for family members, and 1 for myself. If you didn't mind firing an oil boiler in one house when the mercury drops out of sight, a 520 becomes a possibility...but you could forget about adding a fourth house to the mix. At the end of the day the guy selling you on the boiler needs to be accountable for the performance...but you should still do your homework.
lsayre wrote:The KD-12 looks like it would do the job. Keystoker boilers are noticeably less expensive than the others also.
Like anything else, that depends on who you price it from, and what you price it against. I have never run a Keystoker, but I have stood next to one that was running.
They are very simple, quiet, and cheap to fix in the rare event it needs parts. Biggest downside (in my opinion) for a hard-working application is the additional cost of their auger package. Approvals and UL listings are another issue, but it sounds like this boiler would be located in an outbuilding...so that is a moot point.