Due to a heat duct that runs floor to ceiling in the wall, I plan on extending the doorway opening from three feet to six feet and leaving the duct work in place to save time and money.

Over heated?

It might pay to take a heat gun and walk through all your rentals and check their heat on the lights. 

Although your current maintenance is very low because of the newness of the property, you will have to replace the roof, heating system, rugs, flooring, etc. over time so that you should account for it.This is how I analyze the property. 

He traded that for a water well drilling business later on, and soon added a Remodeling and Victorian Restoration component, a Heating and Air component and a Plumbing component.  

In an early multi purchase, I had to replace/install 60% new furnaces due to break down and separating of heat. 

There are issues like; temporary steps to the porch; a bank will likely say the roof needs replaced; the heat is electric baseboard with a pellet stove in the living room (could remove it). 

Then maybe that would correlate with previous posts of heat/cold expansion.

Lack of heat in the dead of winter will be a big issue for most people.

I work for a home improvement company in the summer and I drive home heating oil trucks in the winter.