That looks pretty shot, abused, neglected, possibly frozen at least once and that's all it takes!
I am amazed the gas meter is directly next to this, that seems like a dangerous spot to put a meter!
If it's about 750,000 BTU, you can figure that's about what they originally figured it would take to heat the building, but you don't know know to what comfort level they allowed for 75 degrees or 60 degrees.
Assuming it's about 55% efficient as someone suggested, maybe a modern more efficient unit maybe 500,000 BTU getting at least 90% would be close.
Rememebr too, the all brick uninsulated building has an R value of single digits, if the attic space is insulated and the exterior walls are, and if the windows had double glass it would substantially reduce what's need to heat. If you have to remove/repair the insides of the exterior walls ANYWAY, thats when you can fit insulation in there for little more than the materials cost, and that would be well worth it!
Just Googling for an example to get an idea of cost, I found:
Dunkirk D248A600A20 D248 Commercial Series Electronic Ignition, Gas Fired, Steam Boiler, Taco Pump, 80% - 600,000 BTU
- Steam System
- Natural Gas
- Cast Iron Heat Exchanger
- Vertical Vented
Price: $6,789.99
That's just for the boiler unit, no installation, chimney, piping or radiator/repair/replacement if needed.
So You are looking at at least $10,000 for a unit like that to get it to the basement, set in place, and a start on the distribution systems.
That's not a whole lot but that one system alone is half the cost of what the owner is trying to sell the building for.
Other options might be looking at zone heat/cool rather than central, by this I mean each "apartment" or whatever has their own unit which they control, and which runs on THEIR electric and gas bill, while you can do with a much smaller unit that would cover any public space such as the hallways etc.
Still, you would have to buy those units and install them, and buying say 4 of those or however many are needed, likely would cost about the same outlay as the one boiler in the basement, though maybe you can buy and put those in when an apartment is rented.