The biggest risk I see is one decent size hiccup and they could be toast.

Given the size and scope of things, and "community" one could readily argue addition of organizer and head of a Criminal Syndicate. 

i.e. longer than normal vacancy, bad tenant screening, poor maintenance, skipped rent payments, etc.There could be a handful of reasons why it lost it's ability to cash flow.

As one who personally had a medium sized HML company going into 2007 2008 with 600 loans on my books and by 2010 went through a massive re value melt down all of us in that business got hammered for us we started in 2001 and by 2010 had to basically shut down let go 90% of staff and spent 3 to 4 years hemorrhaging our net worth's to make sure our banks got paid ( which to my personal financial situation was devastating) Most of my peers went out of business completely ( BK) I did not but by 2011 I was basically starting over.

Even though I had a full size W/D on site, I still used this method.....

About the size of a package of crackers, in rectangle form.  

Find a few properties that are your closest competition (similar size, location, finishes).

We own and operate a mid size REI and property management company in Marquette Co. (~300 units or so). 

One of the best ways to start investing imo, especially for young people who can be flexible on location, unit size, etc.3- look for properties with value add- renovate and/or reconfiguring units, maybe adding an ADU in basement or garage conversion, upside in rent increases via tenant turnover due to natural attrition, buy outs, etc.

Gross profit is interesting because it appears to just be the difference in his acquisition and sales price and fails to take into account the other obvious cost of running that business which includes, again according to him, the cost of over 26,000 letter print, mail, post, and the cost of the lists needed to create a mailer of that size.