That was not a great answer to your actual question, but those are the big issues with vac efficiency out side of the unit and the duct work itself.  

I did have to pay an abatement contractor $2,000 to remove the old duct work that was likely covered in asbestos.

Pat:  Guess what the rest of the ductwork looks like?  

The inside is a total rehab floors, paint, windows, hvac (adding duct work), new baths, new roof, adding a front porch, etc.

PLUMBING expansion tanks for hot water heaters, copper scabbed into galvanized pipe and proper venting HVAC - sealed duct work and properly sized for sf.

The only problem I've run into sometimes is that existing ductwork was not exactly "made" for air conditioning so the location of the returns and vents is not ideal... so certainly rooms may not be as cool as other rooms.

One of the unit already has central AC.I’m really interested in HVAC, however no existing ductwork on 2 of the units.  

If the lines are already et/ duct work is done is typically costs about 3k on the affordable end for labor and install.

Is electrical or duct work running through the area that you want this door added?

Central air systems in these homes would have been added later on, so ductwork is often located in a very small attic crawl space.