Forget about the interest accumulation...you're not paying it...as long as you have positive CF, the source for the funds to pay the mortgage, and all other monthly expenses is the tenant's rent.I don't draw a line between CF and equity. 

In high school and college I started reading up on how to invest and how to build wealth, and it seemed that everyone with considerable wealth listed real estate as a source of income. 

Whats your number source of rentals when buying?

I am the only income source in the family and she is a stay at home mom.  

I think you are missing the most critical cost upper. 

To calculate that I would need a formal source that would indicate the useful life of a typical quartz countertop.

There are hundreds of sources for your loan as long as the HOA has money in the reserve account to weather the storm of a lawsuit.

Depending on the website you can run a search for sales on four family properties as far back as their records go.You could also pay for a lists via website's like list source.

Choosing the right market to invest is a critical piece of the equation as some market are cash flow heavy with little/no growth and some are appreciation heavy with negative cash flow. 

My thought would be spend way more time looking at deals, sourcing deals, closing deals, than reinventing a great well functioning wheel.