This is a condensed and over simplified example, of course, but hopefully it answers some questions.

We have a combination of gas furnaces and electric condensers.

Get a reputable AC contractor and you'll get a 10 year warranty on your condenser.

Electrical costs go along as there will need to be a disconnect at the condenser(outdoor unit) and power to each head.

After calling the home warranty company and having a specialist come out, he discovered that the airducts are leaking from condensation.

The inspector does not believe there is any flooding as the hardwood floors are not cupping, but rather that it could be condensation or some level of leaking.

Individual gas forced air furnaces with A/C condensers. 

It gives me a reason to go in and change filters, vacuumed the condenser blades for the refrigerator and just do a general safety and maintenance check.

I did it by bringing rents to market value, reducing delinquency by improving tenant performance, developing and implementing a water conservation program, installing temperature limiting thermostats, developing a a/c condenser maintenance program, installing solar screens on all windows to reduce heat input into units, renegotiated service contracts to reduce cost, and on and on.....  

It looks like you have asked a lot of the right questions here, and here are a few things I might add:Due Diligence on the PropertyWhen were the last major updates to each property (get an itemized list), you'll want to know the age of the furnace/boiler system, condensing unit, hot water heaters and roof.