One had a missing air conditioner (stolen) which I didn't anticipate.  

If you opt for this just check with an electrician first because a heat strip can draw a lot of power 30 - 50 amps.Check out this link to an article on Carrier's website.https://www.carrier.com/residential/en/us/products/heat-pumps/heat-pumps-vs-air-conditioners/Furthermore if you can find a local HVAC engineer or energy calc specialist to run energy calcs for you they can tell you exactly how many tons of heating and cooling you'll need based on the SF of the house, type of windows, type of construction (wall make up, floor make up, roofing etc.), how much insulation is currently installed etc.

They also eliminate the need for additional air conditioners or dehumidifiers in the summer which are less effective.

I'm curious as to the psychology involved with external security measures.  Specifically whether or not marketing 'added security' via #ACCages, Steel Doors, Burglar Bars, Crawl Space Doors, etc. would make a propert...

And to tell you the truth pretty much all manufacturers offer the same warranty the same components the same stuff... 90% of all air conditioners are manufactured by two larger companies anyway.

If i can buy it cheap  enough would it make sense to tackle the project of adding an air conditioner to it?

In addition to insurance, taxes, P+I, landscaping and vacancy rates, be sure to include a capital a percentage of your rental income for major capital events (roof, air conditioner, etc) and basic operating expenses to be sure the investment is going to be sound even when you hit a rough patch.  

Pay very close attention to the condition of water heaters and air conditioners.

A tenant asked me to waive the $25 surcharge per month for a window air conditioner, and I said no.

As an investor I’d rather replace a broken air conditioner and pool filter than having to install new flooring, granite counter tops and repaint the entire home ….