Thing is, this house was poorly remediated, they didn't cut high enough or even let all the furniture, belongs, etc. to air out.

Does anyone have any experience with  this type of forced air furnace? 

The first floor has central air and forced air heat, while the second floor has steam radiators and no AC.My wife and I plan to live here for many years, and would like to either (1) duplex the first floor unit down into the basement and keep the top floor as a rental unit; or (2) duplex the first floor up into the second floor unit and convert the basement into a rental unit (we believe it could be a 2-3BR, 1.5-2BA unit).  

After you address the leak it might be worth looking at additional venting or conditioning the crawlspace with a duct from the air system.

I look at properties all the time and I see the condition of many properties around here that are "professionally" managed where the repairs are unbelievably shoddy, the cheapest indoor/outdoor carpet is used for flooring, they often have galvanized plumbing, and still have sections of knob and tube electrical. 

It was your tenant's choice to sleep in a hotel instead on the first floor where it is air conditioned.

Get Permission to go ahead.2. mini split air conditioner, insulation for garage door pop in options . (1k)3.

With this, fresh airflow is very important to avoid stagnant air and humidity within the home.

I've just moved to the Boston area as I continue to work my profession in the US Air Force.

and I wonder how much they would pay per night or short term how do you figure that out..Most of the Air bnb s I have seen around here are modest homes  or like ADU';s  there are nice ones at the coast..