The cigarette odor and tar (yellowish residue) was found in the heating system furnace and duct work too.

It has a new heater and ductwork.

@Sviatoslav Gulko  sounds like the ductwork was sized just for a heating system .

That is much cheaper than the minimum of $2,000 I have to pay just to swap out an existing gas furnace, assuming there are no alterations required for the duct-work and vent systems.$1,000 buys a LOT of insulation. 

I am also trying to find a product to slip inside the ductwork to the ceiling registers.

There is currently no ductwork in house, so price includes heat/air and ductwork.

In an extreme case, you might replace the duct work. 

Pay attention to soffits, boxes around ductwork and pipes and to the straightness of walls. 

We have slowly upgraded the property by adding in a new driveway, walkway, new duct work under the house, updated the landscaping for curb appeal, installed recessed lighting and ceiling fans throughout the house and repainted all the interior rooms.

We have the intentions of keeping this duplex long-term as it is the start of our real estate investments, but want to know whether it is a wise investment to upgrade to central heating and air in Ohio given that we will have to run two sepearte systems and all duct work in addition to upgrading the electric box.