In the town it is standard to include water.In one building I have a central boiler and old cast iron radiators- in the other I will have a mix of 2 separate furnaces for the downstairs apartments and new radiators upstairs running off the boiler in the basement.

Do you have radiators or forced air in the apartments.Radiators,, all run off same boiler so you'd need to either include heat with rent and call it a day.Only real way to figure out what's up on this building would be to call a heating plumber guy to inspect so, could tell you if you can convert this furnace/ boiler to natural gas.. as well as probably the water heater.. and what cost and energy savings would be.. if you need new boiler/furnace might be worth energy savings to replace.. separate meters for boilers aren't possible because heat system is one system and not individual.. 

The first question I have, since you already have gas service there, is: Is heat delivered via radiators or via forced air through registers?

They could have 1950s or earlier; wiring and plumbing, and probably radiators and no AC. i am not saying avoid the area, just be careful with your estimates.

I am strongly leaning towards doing an electric heating system for the upstairs apartment. currently there's a gas boiler with the old cast iron radiator steam heat.  

What would it typically cost to install a completely new HVAC system to a home with radiators?

Shot the radiator.

They should pretty much always be replacing the coil (this is inside but sits usually right on top of the furnace and looks like a car radiator) when they replace the ac unit.

The first time her “radiator goes out “or she “was off sick last week “ both of you will have a different feeling !

To add a little context...we have a home with working steam radiators, gas boilers thag are 7 or 8 years old.