We put some in the air conditioning duct work as well, I think that helped get rid of the smoke smell that would circulate through that system.

HVAC if none was there, means adding; Ductwork throughout the house, Framing to box it in, drywall, and finishing.  

With the gas line already run into the house, the question would then be whether or not to keep the oil, get a gas boiler, or bring in a gas furnace and run ductwork for forced air.  

Last weekend they were helpers on the HVAC installation , new heat pump and duct work . 

When doing our inspections I look for cigarette butts in the parking spaces, outside the residence doors and patios, in the fireplace, and in the floor vent ductwork.

If forced air, you could line up your two new furnaces with the existing ductwork.

Would love to hear your thoughts   A crawl space with the airhandler and duct work located there helps alot.

Additionally, of course you have to either install a split for the conversion or add duct work to the garage, but did you know most appraisers will not count the square footage if you do not level the floor to the rest of the house? 

. Just from what you described your cure items would be 1) Removing the unapproved occupants from the house 2) having the carpets professionally clean and provide you with a receipt 3) depending on how bad the filter was, having the ductwork cleaned, 4)  depending on how bad the walls are, either you’re asking them to use a magic eraser to clean the walls in the woodwork or paint.

Brian Pulaski my Hvac guy charges 4500 for most brand new installs gas forced hot air and new ductwork.