They're perfect for that type of sq. footage and very quiet and energy efficient, easy to install and don't require duct work.

A hardwire installation would definitely be less expensive than adding a new vent to the room (ductwork, drywall, etc).

When it's time to replace, I would budget anywhere from $2,000-4,000 a piece depending on the size (BTU) needed and to what extent the duct work may need to be modified or resized for the new furnaces.

I agree, running ductwork would definitely add to the cost. 

Perhaps doing a deep clean on the ductwork as well can help?  

If you needed to install new duct work, then I would not consider it. 

Duct work & a/c evaporation coils need cleaning, carpets & padding are usually toast as is most of the door trim the animal uses to exit/enter.

If you mean there is a leak in the duct work that makes the heating less efficient, that is likely a very simple issue to resolve for a few hundred dollars.  

Smoker houses have unique sets of issues like HVAC, ductwork, ozone equipment, cleaning requirements, and quality sealers and paints in significantly higher quantities than normal non-smoker rehab.

I planned to toss the fourth one and go back to a water tank heater since the duct work was still in place.