As a scientist and engineer we do not just "take" a theroy and blindly apply.

If you're a single mom with student loans, a car payment, and living paycheck-to-paycheck, then $20,000 would be devastating and a reserve is critical.

A couple of additional concerns are; -downward slope towards the house ( may be worth getting a structural engineer incase?

For example, "My car broke and I had to get it fixed so I could get to work.

While frustrating for the typical investor, the Blackstone's of the world still have to provide an alpha (outperform the market or other available investment vehicles) to their investors or they will park their capital in another sector or asset.Removing capital from a business is almost the equivalent of removing gasoline from a car, without gas the business can't run.

Typical uses are to pay off credit cards or car loans, and many investors use these to purchase more real estate! 

Do all the houses on the block have a garage to park a car or all have bootleg units?

You will need architectural and engineering plans for the builds.One of your other main points of diligence from the beginning is going to be how will you get water, power, sewer/septic, and potentially clearing of trees and other land issues to get to build these properties.

I would compare it to car repair, if you can watch a youtube video and feel confident changing out a part, then by all means save the money.

Suffice to say, the analogy is doing a paint job on a unique car - but leaving the mechanicals in place without addressing reliability.