It’s interesting to know that the house previously had central HVAC, so ductwork is already in place (perhaps just needs cleaned?).

Assume it is a full-gut job, needs heat, air, duct work, and electricity, 4k sq ft, higher end finishes. $100/sq ft?

All your duct work , electrical, and some plumbing could go up in those runs now.If the house is not a lot of square footage you could do a split AC/Furnace. 

I recently had my furnace moved into the attic due to some water leakage into our in slab HVAC ducts that we thought was coming from the old furnace.So my question is this: now that the duct work is abandoned, what is the best way to seal the duct work off? 

I recently had my furnace moved into the attic due to some water leakage into our in slab HVAC ducts that we thought was coming from the old furnace.So my question is this: now that the duct work is abandoned, what is the best way to seal the duct work off?

One of the closets "has exposed asbestos material on ductwork that should be professionally encapsulated or removed.""

Tune ups, coils are cleaned, refrigerant fine, even replaced capacitors, topped off the attic insulation (just in case) AND had the duct work sealed. 

having a situation in a duplex without ductwork and thinking about going to a PTAC unit (similar to what you see in hotels) inexpensive, cheap to replace, easier to work on than a mini-split system (and in the north mini-splits do not work in extreme cold) 

They go into my unit and basically tell me they really recommend replacing the unit because it’s leaking oil as well, has primitive safeties, the ductwork wasn’t even screwed together and there was a crack in the heat exchanger and to make sure I had new CO detectors… I knew the unit was very old and did not look the best by any means and kept a mental note that it would probably need to be replaced in the near future but it was still pretty operational so I wasn’t too worried.

The big thing for me was the lack of separate HVAC systems with shared ductwork.