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.

There are no existing fans, so I will need to run the duct work, and install the vent to the exterior.

You will also need to consider how to access the hook-ups from the rear of the units (dryer vent hookup could be a pain since they usually are low - look into some of the ductwork alternatives at the big box hardware stores for some ideas here).

There is this thing about range hoods needing electricity to power them ...Then there is also an issue as to whether it will be vented to the outside (think hole in the wall and duct work in kitchen cabinets and the $$$ add up), or whether it will just vent into the air above the hood inside the kitchen.It's not the price of the hood itself so much as the installation ...

Get the duct work and HVAC professionally cleaned.