Economists credit a mix of factors including ultra-low mortgage rates, increasing demand for space, and an emerging home-buying demographic: millennials.

A couple of factors are at play.First, rents trail purchases because it's a less efficient market. 

I'm not factoring in appreciation, equity, or 1031 exchanging into better properties, rent raises, or promotions at work, so it can go more quickly than that, especially if I can figure out ways to use OPM so I can scale more quickly.But, similarly, appreciation would mean that acquiring such houses would take longer, and that's assuming I can find properties that actually cashflow $300 after everything, including capital expenditures, repairs, etc.

What size is the multifamily, this could play a factor.

I am not sure why rebuilding as a single family if destroyed makes a difference since my mortgage broker confirmed I would qualify for the property without factoring in the rental income (i.e. if this was a single family at the same purchase price I would still be able to afford the property).

If you did, I would consider this a sunk cost and not relevant when factoring to sell and the 'ultimate loss' you will incur. 

I wish you well but I'm curious how this factored into your decision? 

Keep in mind you should still account for maintenance, capital expenses and future improvements (which are all accounted for in commercial property along with vacancy factor)I know this is a lot of information but success is in the details.Final point, if the numbers are right this property should qualify for rehab and long term financing.

The BRRRR factor in my thought process is that leveraged more quickly. i.e. with option 1) buying more turn-key properties without forcing appreciation and refinancing (BRRRR)...it may take me many times longer to acquire the same value/# of properties, vs using option 2) the BRRRR method.

In both cases, syndication would be needed with a securities attorney.Here is the list of promissory notes recognized by the Supreme Court as NOT being securities is a compilation of nearly random examples:Notes delivered in consumer financing.Notes secured by a mortgage on a home.Short-term notes secured by a lien on a small business or some of its assets.Notes evidencing a “character” loan to a bank customer.Short-term notes secured by an assignment of accounts receivable.Notes that formalize an open-account indebtedness incurred in the ordinary course of business.Notes given in connection with loans by a commercial bank to a business for current operations.And the determining factors for how a court will judge it to be a security or not are :Whether the borrower’s motivation is to raise money for general business use, and whether the lender’s motivation is to make a profit, including interest.Whether the borrower’s plan of distribution of the note(s) resembles the plan of distribution of a security.Whether the investing public reasonably expects that the note is a security.Whether there is a regulatory scheme that protects the investor other than the securities laws (e.g., notes subject to certain banking regulations).Again, consult an attorney.