Help with decision to add central air to a duplex in chicago

[Jim Cellini]

Hello bigger pockets.  Quick question pertaining to value of adding central a/c.  

I recently purchased our first house hack with a physician loan in Chicago.  Our home is a duplex 100+ year old house in an up and coming area on the northwest side of the city (logan square).  We are currently renting the upstairs for $2100/month (3 bed 1 bath).  We are considering adding central air via a "high velocity" system such as spacepak or unico.  I've had some licensed and well reviewed contractors come out and we're getting estimates in about the $20,000 range for the install.  

My decision at this point comes down to whether or not to do this or to use that money to buy a 3rd investment property. The most objective way I can think of to decide this is basically "what is my ROI". I know about what I would get with an investment property in my market in GA (12% cash on cash is doable). My questions are:

1) Anyone familiar with Chicago, how much more could I reasonably get in rent with central air vs. wall units like what we have now? If I can find this out I can compare ROI's between this move and investment property. Generally speaking, how important is central air in chicago anyways?

2) Does anyone have experience installing such systems into their home, specifically an old home such as mine? Any factors to consider with their usage and installation?

3) Anyone have any references for a reputable hvac contractor in chicago that installs high velocity systems such as those mentioned above? 

Thanks for any help!  Truly I appreciate all of it. 

- Jim