My unbiased wisdom? Well, it will depend on who’s going to have (if anyone) a final word in accepting this installation (e.g. Chicago Building Department, Peoples Gas).
The code says nothing about having (or not) the forced air furnace installed in a kitchen or enclosure requirement (in general), however, it requires the appliance to be protected from damage. Kitchen may not be a dangerous area for a furnace but since the apartment will be rented you have to think about the tenants, their kids, etc.
Furnace service panels have no padlocks and getting them off the appliance (in some models) is extremely easy, even for a child or by accident. Therefore, even if this may not be required, I would definitely put it behind a lockable door. Clearances per manufacturer’s installation guidelines / often depend on the appliance’s type and model.
Combustion air is not an issue if all those furnaces are direct vent type which means they use only exterior air for the combustion process:
high efficiency, condensing furnaces with 2 PVC pipes (exhaust and intake)
single PVC with double wall
and around 80 % efficiency furnaces with two metal pipes (some older models, I don’t think they make them anymore).
If some of those furnaces are using interior air for combustion you may have a problem. The square footage of those units may not be sufficient to provide enough fresh air to support clean burning.
Second issue would be the cold air return but I’m not sure if I understand its current location (kitchen cold air return is not permitted, living room may be OK, or both (seal the kitchen one)), which must be a minimum of 10 feet from any open combustion source (furnace combustion chamber, water heater, and fireplace), unless located in a separate room.
If some of those regular type furnaces are venting through the side wall… well, you will need a chimney or probably replace them with new high efficiency furnaces.
If starting over… I would go with small high efficiency units, dual PVC (vent/intake), reuse existing air supply ducts, install air return wherever most efficient / permitted, and build a closet around them (kitchen or any other location that makes sense, hard to say for me because I can’t see the layout). If you have a new electrical service and enough power to handle electric forced air furnaces… well, for as long as some of the existing gas ones can be salvaged.
Let me know if you have more questions.