Clean air-conditioning unit.?

It has forced air gas heat. 

I have walked in to one property in the middle of winter here I believe the high was around 20 degrees that day, and apparently  the owner told the tenants he was installing a new package unit prior to the foreclosure, and pulled off the unit leaving the giant opening to the outside where the cold air was running through the duct work through the entire house.  

This would imply from about $22k to $27K in annual net income before any financing. w/ a 7% cap rate (picked out of thin air, go ahead and pick and calculated your own..), this would support a total cost of about $320-$380K, or $160-$190/sq ft.

I was able to run flexible duct in a small attic space in one of my duplexes to get forced air heat and central air installed. 

It's more like a partnership where principal reduction is required as if you were paying out profits, the lender will always want to limit involvement over a long term, there isn't any reason to prolong the aspects of higher front end risks for the rate charged, especially at current rates.

I am a IICRC certified mold remediator and my best advice is air samples outside, in crawl space and main living area.

If we had to choose between central air and a finished attic (which would probably be considered a (walk through) 4th bedroom on unit 1, which is currently a 3br.

Back in my Air Force days.

You biggest, common items that would need to be repaired are the air handler unit & the compressor.