Thanks in advanceYou risk a potential securities violation when you start "soliciting" investments from 89 strangers whose names and or personal information you obtained from some lady at an estate auction and probably without their consent.

To obtain this, the units typically have to be in very high need of a rehab. 

Which are the main variable I may working on to obtain better cash on cash?

Hustle can obtain knowledge and apply it.

From the Subject Property The rent for a 2, 3, or 4-unit property, where the borrower resides in one or more units and charges rent to tenants of other units, may be used for qualifying purposes.The Mortgagee must verify and document the proposed Rental Income by obtaining an appraisal showing fair market rent (use Fannie Mae Form 1025/Freddie Mac Form 72, Small Residential Income Property Appraisal Report) and, if available, the prospective leases.Where the Borrower has a history of Rental Income from the subject since the previous tax filing, the Mortgagee must verify and document the existing Rental Income by obtaining the Borrower’s most recent tax returns, including Schedule E, from the previous two years.

After the second time the bank tried to slow my progress I decided to seek and obtained seller financing.

But don't underestimate the time it will take to find a lot, obtain building permits and clear all legal requirements, build and landscape. you are probably looking at 1.5-2 years from first steps to completed and rented.

So to summarize - it looks like the due diligence is mostly the same, however there needs to be even more caution taken when checking and verifying the payment history, so make sure to do things like obtaining bank statements and even an estoppel like agreement.

Can't believe Brandon Turner offers about 50% less and obtains the homes. 

conventional logic would be that you spread your risk over 5 doors instead of one. the only part that might be more difficult is obtaining financing as most mf lenders will require a deposit of 20% or more. good luck!