Ventilation systems: Furnaces and heaters will either need to be replaced or cleaned throughout all ductwork.

Removing radiators and pipes and installing HVAC and duct work.

New roof, duct work, paint inside and out, extensive drywall needed, broken windows, replace all light figures...we're talking a lot of work!  

More efficient, can control temp in each room, no duct work in attic, etc...

But unless you can separate the heat, meters are useless.Heat: Have professional look at the duct work.

Then some baffles in the duct work move and then diverts the air to the other level and the system then begins to work on the 2nd command.We tell the guests how this works & inform them that while the system may not immediately begin to adjust the temperature in their apartment if it is working on an adjustment for the other apartment, it will eventually. 

I see the point about the old components as an item to negotiate as part of the offer price instead of after the inspection especially since it still works (surprisingly since it's a gravity furnace from 1952), however I will have to replace it and it may or may not have asbestos and will be costly since the ductwork will need to be replaced as well.

Adding central heat and Air to a 12 unit could cost a lot of money and be very disruptive to your tenants unless duct work is already running through the walls.

@Patrick@Patrick Collins- new roof, hvac unit (vents and ductwork already there), refinish floors, carpet, paint, tile bathroom.

The home has a new furnace and ductwork along with updated electrical, the ac was replaced in 2012.