I don't think you will be cash flowing much of anything when it comes to your home maintenance expenses, time and energy etc etc. 

(They can't provide insurance on any fines passed on to me in case my landlord gets fined by NYC for having illegal short term rentals in the building, so this is a risk I have to bear)Question is:1) Should I bite the bullet and take out the insurance policies on all of my units?

Do you and she really want to put your energies into taking people to court?

Either a bullet or a firecracker.

Not having a long commute allows you to save a lot of time and mental energy that you can then put into a side gig or getting up to date with real estate knowledge.With that in mind, I think house hacking by renting out a room in a single family house or condo type of situation should work well.

After looking at the pics, if it were me, I’d bite the bullet and replace.

@Kenny Durrant  While I don't make my buyers sign any agreements with me until we are writing an offer and I have to, if I knew my buyer was working with many different agents, I wouldn't focus nearly as much of my energy in finding that person a deal. 

If the unit is well-sealed(you can check for drafts by walking around with a candle and seeing if/when the flame blows(super-cheap way,) or get an energy inspection(more $$)) and insulated(double-pane windows at least, proper wall insulation- and maybe ceiling too?

I have about 90K in my primary residence and my wife and I are thinking about bitting the bullet and moving in with family and selling the home to save some money and get an investment property. 

If it were my house, I would bite the bullet and do the install. 1, it will make the house safer and more comfortable, 2, it's more attractive to potential renters, 3, it makes the house more valuable (as you already mentioned, central HVAC is common). 6-9k is about right, I've had 4 or 5 systems done over the last few years and I've paid everywhere in that spectrum depending on how big the house was.