How do you determine heat loss? Barn will have a frost free hydrant outside but not tied in to boiler.
Do a google search for online heat loss calculator. As there are several, suggest it might make sense to use a couple of them to compare. You will need to know all 3 dimensions of the building, how much insulation, how many sf of windows, etc.
On the makeup water...since you are choosing a boiler as your heat source, I would suggest it makes the most sense to run water into the building for makeup to the boiler. That's what I did in my garage. One less thing to worry about....or possibly forget about and lose heat and freeze up/possibly destroy your boiler.
Phil