I think the plate boilers are more tolerant of improper use and off-season storage...but they can still rot out if not properly cared for. I wouldn't pass up a nice looking tube boiler if you find one. In this particular case the boiler was previously used for steam...it wouldn't surprise me if the steam system had leaks and required make-up water on a regular basis. The oxygen in the fresh water is tough on the boiler. If the boiler was shutdown for the summer without being cleaned and treated with a corrosion inhibitor (or kept warm with a light bulb), that is even worse.
If you want long life out of a boiler....fix any system leaks immediately, you do NOT want to be adding fresh water on a regular basis. Make sure the tankless coil gasket mounting bolts are "snug". Every boiler I have installed got anti-seize on the tankless studs, and the first time it gets shutdown I check the nuts to make sure they are snug...just enough to put even pressure on the gasket (do not squish the gasket out). If you don't run the boiler through the summer, make sure it is thoroughly cleaned in the spring, sprayed down with WD40 or something similar, and put a light bulb in the firebox.
If you use the tankless coil, make sure there is no water flowing through it when the boiler is not being used...otherwise the water will cool the boiler and cause condensation in the firebox.