After some deliberation, I decided to let it go and let them get away with February for free in addition to all the months they'd been owing before that.Now that the deal had closed, I gave her a choice on the heat: either have handymen busting up the floors and ceilings for a couple weeks to lay new duct work, or install new electric heating.

In my estimation to do a tap in and run the lines + add two furnaces + ductwork is going to cost around $9,000.

I am just afraid that if I need to put a new roof on and new wiring and install an Ac system and Duct work that it’s not going to leave much for everything else.

However, I continue to find more pieces of insulation appearing in the ductwork several months later. 

Major duct work is needed and yah knew that.

(The duct work runs the length of our living room to outside wall; approximately 15’).

Inspect duct work for dryer vent - tenant says it's disconnected and not connected to vent location in roof/not secured to the wall10.

im considering getting an electric furnace because there once was a forced hot air heating system and the duct work is already installed and there are vents all throughout the house.  

Any suggestions on things I can do to reduce the noise between preexisting duct work?

You've got to not only add a heater for each unit but, the associated ductwork/piping to get heat into each unit, the electric for each heater, and the gas for each unit needs to be separated.