With rents being around $600 for those units, this suggests the more budget end of the rental spectrum and for that AC unit type is not much of a concern.

If you rehabbed a property in this neighborhood you run a much higher risk of not getting the appraisal needed to cash out refi your initial investment due to the small number of comps at the higher end of the price spectrum. 

@Aaron K.You should examine your holistic taxes paid across the whole spectrum (income, sales, property, etc) before you make a determination how friendly or unfriendly an area is.It's generally accepted that when a state reduces (or eliminates) taxes on one revenue source they must raise taxes on another revenue source to compensate. 

@David Chen @Scott Hibbert I'm currently under contract for my first house hack on sort of the opposite spectrum from yours in the Villa Park Neighborhood.

I'd suggest learning how to analyze a deal on both sides of the spectrum so that if an opportunity arises you are knowledgeable enough to seize it.

We have owners under management on both sides of the spectrum. 

On the other end of the spectrum, you find the {near}vacant building, buy it for nearly nothing, and give it a deep retrofit (energy efficiency improvements, layout improvements, update core systems, and refinish throughout).  

They were likely slobs but it is not really that bad. 3 years, within the spectrum of normal ware and tear by tenant/rental standards.

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. 

He is the Plano area, and has a very diverse background that ranges the spectrum of real estate investing.