If you intend to hold onto the property, or are just inclined to do your rehabs correctly, get a unit that is properly sized for your ducting and sq footage.FHA HVAC equipment that is oversized for the installed ducting is prone to shorter life span due to short cycling and potential cracks in the heat exchanger since the ducting will not allow the proper amount of air to be moved across the exchanger.

So, like a window air conditioner, but installed in the wall instead of a window?

@Jordan Sutherland might be able to provide some additional input though. 

You might ask your bank how long you have to own the duplex before you can get an equity line of credit on it - you might be able to finance the systems through the HVAC company, pay that note for 6 to 12 months, and then pay off the HVAC company's 18% loan with 6% money from your equity line.Right now, every HVAC company is busy fixing and swapping air conditioners. 

Then the negatives: Train tracks, outdoor gun ranges, air strips, school zones, kennels, flood zones, busy/noisy roads, industrial activity, proximity to odor-generating things like a landfill, paper mill, fish processing plant, etc.In other words, unless you have another reliable source of this kind of detail, USE A REALTOR.

Picking numbers out of the air for expenses and looking to hit a certain goal isnt any different than making up a rent number to inflate the income on the other side of the equation.

I am a newly married Air Force Family Medicine Resident Physician at Eglin AFB, Florida. 

Our newest acquisitions are in the Space Coast of Florida, which is where my husband was stationed with the Air Force (at Patrick, AFB).  

I’ll have to investigate Air BnB regulations some more, but it sounds like Henderson may be promising.

Would anybody with experience on retrofitting an older duplex with central heat and air feel like offering some advice on how extensive that project can be?