One of my units doesn't have central air but rather has a few AC units.

Unfortunately, air conditioning is not considered a health or safety issue in Texas.  

As you can imagine, some guests are cleaner than others and as a result we sometimes have issues with bugs.In the past the property manager has set off a fogger after each person leaves, but honestly that's something I'd like to try to avoid doing as much as possible.

Yeah well NorCal is literally ON FIRE.. the air is so smokey, But, that would be off topic. 

I'm 10 miles in the air looking at your situation, so take it with a grain of salt.

The Tenants kids spill koolaid ( poor people love koolaid ) and they let it sit or try to clean it up themselves by a wet towel instead of immediately using a steam cleaner .

Current furnace was installed in 1987 (very old) and it’s forced air furnace (no radiators)Any recommendation on what I should do?

Macbook AIr is awesome for the price.

Or, they will develop a box that just sits there and blasts out cold air into a house which will reduce the summer heat to a much more livable temperature.

I'm 25 years old and on my first assignment with the Air Force in Tacoma, WA.