With his high-mass radiation even a 150K unit probably won't short-cycle itself into an early grave- it'll probably go at least 15-25 years, whether you want it or not, (and maybe even 40) but it'll run at ~70-75% net efficiency rather than it's rated 83%/whatever AFUE.
At his load, short of a mod-con I'd rather pay for a cheap 50-60K cast-iron beast, (preferably a forced draft side vented unit, since the chimney is obviously way oversized for a smaller boiler) and have the thing operating on at least SOME substantial duty cycle, living on the knee of the regression curve rather than on the shin. It doesn't take a whole lot in the way of near-boiler plumbing to set it up for lower temp at the radiation (== lower distribution losses, steadier radiation & room temps) without compromising the boiler with condensing return water temps.
OTOH, the excess standby losses of the 5x oversized unit mean the boiler room will stay warmer- it might even be the warmest room in the house, so maybe you could make the boiler room the living room, or maybe even the sauna !? :-)