, foundation (a few cracks in drywall in master bedroom) and possibly duct work (all registers have been removed and there are cobwebs and junk visible ( may just be part of winterizing?

Considering easy access with crawlspace below, and as long as there isnt a bunch of ductwork or plumbing in the way, this is quick, easy, and cheap.As long as the planking is sound, there shouldnt be much give between joists.

I have a unit where they placed the forced air furnace and duct work for the 2nd floor in the attic, and pulled the power and gas supply through the chimney.  

You might have to replace the flooring, clean the ductwork, seal the walls with shulac or Kilz etc....  

I've heard of the typical stuff (copper piping and wiring, ductwork, etc)...

One rental I had melted crayons in the light fixtures in the ceiling and in the duct work and they removed the wall paper. 

Often times in the first contact the prospect asks if there is oil heat,  then they decline to even look at the place.We've done about a dozen conversions to gas heat ranging from about $3,000 to $6,500, some included duct work, gas hot water heaters and central air.

It is my understanding that they have developed a system for both heat & air that basically has no duct-work.  

Everything was taken out of the crawlspace too - furnace, ductwork, everything gone. 

I don't know about ductwork, but they needs new heat pumps.