I think you might inquire about just down zoning to RM-12 just to be consistent with the present duplex use.

My real estate background consist of two flips.

Shortly before  hitting that goal though, you will likely feel a bit overwhelmed as you balance your full-time gig with 6, 7, or 8 properties How are your current investments performing?

Not saying a roofing contractor would have been cheaper (I am sure he would not have been), just think it's a VERY BAD idea having someone that's not an insured contractor performing repair work....especially on a roof.  

Unless your private lender is so small that they don't mind sharing their bank account information with every realtor/seller you are ever going to work with, not many of us are going to give you our bank statements, and the fact that you have someone else's bank statements would not indicate to me as a seller that you have the capacity to perform to a cash offer contract you are asking me to sign as a seller. 

The contributions to a Solo 401k consist of two parts: 1) Employee elective deferrals, which are limited to $18K (plus $6K catch up if you are over 50).

I am profiting by performing the repairs(contractor) and lending the money to do it.This would not geared towards motivated seller, but just sellers with houses that need work.How can I create a model around this idea?

I also agree with @Curt Davis that Memphis is a consistent market that will be attractive for many years to come.  

So a bit vague I know but it will take a while of consistently working a system to get to where you want to be with this.

The mortgage officer needs the estimate by the contractor that will perform the work once the loan is closed.