Most homes in this area (South Carolina) have central heat & air HVAC with duct work.

For example, if the house already has a furnace and ductwork, then adding central air won't be as expensive as installing a whole new HVAC system if the house curently has a boiler.

No one except the mold remediation salesman is onboard with paying $1800 to clean the duct work.

Do I need to remove the drywall, insulate the ductwork, and patch it back up again?

The HVAC system is a unit of property and in your example, all 50 HVAC units plus the corresponding ductwork comprise the HVAC system as a whole.

HVAC efficiency in ductwork puts it first.

It also goes into the duct work etc.

Fans will only circulate hot, humid air into a cooler, drier crawlspace, creating condensation and causing pipes and ductwork to sweat - you're simply adding more moisture!

Changing the air handler or installing a core in the plenum is also the time to clean the existing ductwork and seal all leaky joints that can be accessed.If you do not already have forced-air in the house, then ductless AC or heat pumps are worth a look.  

The SmartBuy offer is a two year warranty on HomeSteps purchased properties, and includes electrical, plumbing, air conditioning and heating systems, as well as ductwork and many major appliances like water heaters, stoves, washer and dryers, dishwashers and refrigerators.