What’s got me considering these systems is due to the fact that the under slab ductwork on the downstairs units is rusted out and we were going to be installing a new overhead supply duct system the would require a bunch of soffits to be constructed to conceal all the new ductwork.

You can often just replace the defective part like the compressor.I have had AC put into a house with duct work for less than that.I believe there is Real Estate Investment club in Charlotte (maybe Asheville) you could ask them for a reference for someone that is investor friendly.You definitely need a second opinion.

His tech had been out there 4 hours trying to get the duct work to stay together. 

I talked to a guy who has property zoned for commercial. the surrounding buildings sold for average $135,000+ and he is asking $60,000. it has new wood floors, a/c, new electrical outlets, new ductwork, etc!!!

We later added duct work and there was no question on permitting as we rehabbed the inside as well.

All the duct work was in the concrete on most.

HVAC/Duct work is a wild guess

Some of the major rehab involves Siding, portion of roof, new sidewalk, splitting electric (new panel boxes and meters), heating (new furnaces and re laying of duct work), water (new water heaters), portion of broken sewer pipe replacement, replace electric wiring to 2 units, new kitchen, bath, flooring, windows, doors (interior and exterior) in all units. 

All penetrations for pipes, duct work, wiring or anything else much be caulked with fire retardant material.In many areas a condo separation will require a subdivision plan, with engineering, blue prints and municipal/county review and approval. 

The problem is that I think the smoke has permeated the HVAC duct work as well.