You really need to start with a heat load calculation to know where it really lives, using your 66F indoor design temp and whatever the 99% outside design temp is in your area, and the U-factors & areas of the different exterior wall/ceiling/window/floor/door assemblies. A 15,000 BTU/hr heat load on a 576 square foot addition would be an unusually high number- it could easily be half that (or less.)
Electric boilers are far more robust than electric hot water heaters- count on it. They're designed to handle higher flow, and they're designed for operation at a high duty cycle. Your presumtion that they are more complex under the hood is mistaken- they are in fact simpler, more rugged beasts than a electric hot water heater, and can be operated over a wider temperature range. The controls are different reflecting the different type of use, but they're still box o' rocks stupid, nothing like a modulating condensing boiler or a tankless hot water heater, but pretty much everthing about them is designed for high-reliability.
The average 3 zone hydronic zone controller has more (and more sensitive) electronics in it than an electric boiler.
The failure modes on electric boilers are a bit friendlier too, with very low probability of a leak or flood.
What does propane cost, and what does electricity cost in your area?