Questions I have is if it makes sense from an ROI standpoint, as my several pro forma calculators don't really factor in these 'family' variables with what my Dad is proposing:Here's the scenario: Purchase duplex of $350k - $400k.

-you don’t have to pay interest on a large fixed sum of money until you find or are ready to pull the trigger on a deal-it allows you lock in some or all of the amount at a fixed rate so your protected against rising interest rates ( given the current market conditions I believe we will see a couple more increases from the Fed this year which will increase borrowing costs for any variable rate products)- See this attached article that does a good job of simplifying it https://www.investopedia.com/mortgage/heloc/fixed-rate-option/

Banking on variables that you have no control over is speculative.

Based on this the estimate of 10% cash on cash seems unattainable (I would be interested to see how you're putting together your numbers to get that percentage due to the variables; it seems the ability to put a low enough amount down to meet that number would equate to the addition of PMI and kill the cash flow on every listing I've looked at in my desired neighborhoods).

You mess around with variables even a tiny bit and you can go from negative to positive cash-flow in about 30 seconds :-)  And, not for nothing, I've caught myself doing it more than once when I've been working my pro-formas. 

Will Glaeser So you have a specific set of variables in your favor for Area A and nebulous desires in Area B.

There's a lot of variables in this question.

There are great variability with different companies.

This picture will answer your question for cap rates in Multis in Memphis this is before leverage. and I don't really go by "cash on cash" because that is all variable on your leverage: down pmt, rate, and length of loan term etc...

Maybe what doesn't make sense to me is (if all variables are equal) why you wouldn't install solar, get 12 months of financials to show the benefit, and resell it.