I had a company come out and professionally clean the duct work ($340, and the company told me that the ducts were clean).  

I still may need to spend about 10k to build trench/french drain to divert away water and about 10k to upgrade old hvac and 10k more if i decide to move hvac from crawlspace to attic by putting new duct work not sure what other major items may come at what time)Now to sell i have to pay 10k to realtor.So my bottom line with just these expenses 226k (not to mention 20% interest i am paying on credit cards for 50k which i used for down payment to hard money lender who gave 0% APR and now increased to 20%)One side 1 year lease tenant pays 915$ with one year lease ends September 2021other side month to month 850$ having hard time collecting rent these days as she cannot speak English and introduced husband who is communicating with me(who is not in original lease and does not want to come to lease says not staying there but seems he stays there) giving all lies and hard times.

Also, when replacing a furnace you don't replace the ductwork.

The AC and ductwork are both brand new, but the house was originally built in the 50's.  

That said, I am trying to determine how much to assume per unit going from a central boiler to furnaces, including the duct work, for +/-75 units. 

The increase in static pressure can cause problems with the equipment if the duct work or blower cfm is not sized correctly.

Plus lots of things with the ductwork need to be done properly, thus the need for professional installation.Smoke detectors:1.

He installed a new 80% furnace in the uninsulated attic & ran the uninsulated ductwork up from the furnace & then down along the roof line into the various ceiling vents. 

Tech tells me the unit is functioning fine. no issues but I have some leaks in the duct work in the attic and that is the likely problem.

Will depend on if you need/want to run the duct work for the venting and how much of a PITA that will be.