More Q s
From your comments I think that you live in an area where air cooling is needed in the warmer months. (This being winter, warmer seems a llooonnnnnngggg way off [LOL]).
1st. Q is the bldg insulated? 50K BTUs sound extra large for 1500 sq. ft. and expensive to heat. Individual heaters for each apt may be the best way, something like a Toyo heater w/direct venting to outside; since you have gas piping already installed, and they are fairly inexpensive.
Where I live (Ketchikan, Alaska) everybody uses oil for heating fuel and I see homeowner after homeowner switching from boilers and furnaces to these units, and their fuel bills are a lot less.
2nd. A house built in 1912, has lath and plaster for wall and ceiling finishes, and if you're considering duct work, how are you going to hide your distribution piping? There is much less repair work in small dia. tubing such as copper for hydronic heating and installation of apt sized air conditioning units.
Plus, you may be able to use a larger boiler, and zone controls (with on-time meters) so you know what to charge for heat.
3rd. I am not trying to throw a wet blanket over your project, and I wish you good fortune in doing your project. I am posing these Q s to let you know that any project in an old bldg such as yours, is a lot more involved than you envision. I would look extremly hard at anyone who says "No problem, we can do this in almost no time, and very little expense". The next thing you know, you're neck deep in the swamp, and alligators are chewing on your legs. As we all know, it's really difficult to keep your mind on the project of 'draining the swamp'.
All the best,
Rudy
EAT "WILD ALASKA SALMON" IT'S GOOD FOR YOU AND YOURS!!