They are also getting a seller's concession which will keep their acquisition costs down.The deal will cashflow from day 1 after debt service and ALL operating expenses. 

This article gives a very helpful starting point.https://www.biggerpockets.com/blog/core-four-real-estate-teamThere are a lot of markets that do have a large selection of MFH that cash flow.

(especially for those involved on this website)The reason I mention if it matters on this website is because most are real estate investors(long term buy and hold) and will most likely operate at a tax loss which doesn't have an impact on taxes.The second point is, will you invest or not invest in a good investment because of what your tax rate is?

Turnkey operations are an excellent way to minimize headaches and begin Cash Flowing the day you purchase your property.

I can give you insight to their operation

then everyone would move to next apartment.These are index-based of core-apartment from all biggest apartment owners in US.

Unless apartments are just not as marketable, then a multifamily building is likely the most efficient for construction expense, operating expense, and rent/SF.With a building of only 12 units, I doubt you'll see any time-based advantage by phasing in duplexes.

I am trying to figure out the fairest way to go out about this so we can have it written in the operating agreement.

You still need operating money to buy flips and long term rentals.

On top of that, you’ll want to budget for ongoing expenses like property taxes, insurance, operational costs (such as utilities and property management), and maintenance.