That has now continued with a purchase every 2-3 years, which has essentially added a third salary to our household. 

I feel that A/C in the summer falls under the landlord's responsibility to provide essential services.

The following is a summary of key elements to achieving that successfully.1. 

Frequently, places will get stuff on custom order that either the buyer returns or never picks up- and you can snag it for a song.Way cheaper to stockpile essentials on the low and then put them to use as needed.

Perhaps the others left out something essential like a missed payment or something.

You don't add lending restrictions to ease efforts generally required of a lender, restrictions and covenants should have a risk management basis or element, not necessarily a profit motive, interest is to cover profits over these usual and customary risks, not high rates and then more restrictions.

So at the end of the day, there are essentially 2 issues to pay attention to  Debt to income ratio, and how carrying a balance affects your actual credit score.  

-Market: Here are essential macro requirements: RequirementsPopulation100,000 plusPopulation ChangeAny increaseUnemployment RateUnder 8%Median Income$40,000 plusMedian Home Cost$100,000 plusHousing VacancyUnder 20%CrimeUse Judgment-Tenancy: Look out how long tenants have occupied the lots. 

Next, is there any legal way to essentially do a form of cash back to the buyers to cover the cost of the repair?

You need to be sure you are getting at least that $600 on that second unit.Youre paying essentially $100,000 and will get "hopefully" $1,050/mo in rent.