Because if you depend on your tenants for maintenance then you are asking for your houses to be condemned with black mold, your lawn to look red as ham and coils in your fridge and ac to look like the abdominal snow man.

It currently has a metal tin roof. 

Metal is not an investment.

Our company was looking for a small multi family fixer upper and this met all the criteria and it was in an A+ location.

You would have to go through the trouble running a metal flue, when you already have PVC right there.

-central air register to the room- on the attic side, the sheet metal is wrapped in some kind of joint compound white stuff.

It may just be the thermostat, condenser unit going bad (coils leaking, etc), perhaps its out of freon, or maybe even bad fuse or solenoid.  

"It looks like it's broken because someone was putting cigarettes out on the coil". 

It is an iron on strip and you just use a regular clothes iron to put it on and then clean up the edge with a metal file. 

@John Collins  Somewhere they should be a metal box on the wall that houses all the electronics.