I figure it would cost me $15,000 to $18,000 to rehab including new vinyl siding, heat/ac, and creating a sun room from the current large covered and screened deck/patio.  

The thing is you can go on and on and on about these rules...Do not strike a match near the gas stove and oven.Do not jump up and down on a bed while the ceiling fan is running.Do not park in your neighbor's designated space.Do transfer or retrieve your clothes from the washer and dryer promptly.Do lock your doors when you leave.Do not leave a burning BBQ grill unattended.Do clean the lint screen on the dryer after use.Do not puke directly into the community waste bin when drunk.When your neighbor's dog drop heat on your door step, please resolve the issue with the neighbor, do not smear such dog poop over his/her door knobs.......This list can be endless though. 

For instance, the A/C inop, could be anything from a bad coil to a $15 contactor. 

From researching this topic, one thing I know I'll need to check is the lowest incoming water temperature, so I'll probably wait until February to try to capture the lowest incoming water temp, so I can choose the correct size tankless water heat.  

Due to a heat duct that runs floor to ceiling in the wall, I plan on extending the doorway opening from three feet to six feet and leaving the duct work in place to save time and money.

Over heated?

It might pay to take a heat gun and walk through all your rentals and check their heat on the lights. 

Although your current maintenance is very low because of the newness of the property, you will have to replace the roof, heating system, rugs, flooring, etc. over time so that you should account for it.This is how I analyze the property. 

He traded that for a water well drilling business later on, and soon added a Remodeling and Victorian Restoration component, a Heating and Air component and a Plumbing component.  

In an early multi purchase, I had to replace/install 60% new furnaces due to break down and separating of heat.