I'm not sure which part is unclear since I presented several concepts.
Rental property is a cash flowing asset. The asset could be anything, a toll booth, an oil well, a fast food restaurant... The net in the financing model is simply arbitrage. That is all. It is the difference (or spread) between the outflow of your debt service and the net income after expenses.
I suspect that your arbitrage is a lot smaller than you realize. Ex-landlords have a better idea what kind of net there is on a rental than inexperienced new investors. You might present your analysis to a burned out landlord for his comment. I think you'll hear that you have omitted a great many items. Usually it's vacancy, cap-ex, or plain old maintenance.
If you really want to push through and do a deal (even as a learning experience) don't saddle the deal with 100% debt financing. Lower your risk by taking on an equity partner for the down payment who also thinks that rental income is respectable. If you offer him/her half the profit, it sounds like you are incredibly generous. My hunch is that if the asset nets 5% on his down payment, you will be very fortunate. If you were to offer the investor 2.5% interest on their money, they would probably laugh at you. But the result is the same.
One data point that is missing in this so far is retail price for the duplex. Is $125K retail market value? What is the expectation for real estate appreciation in BR for the next 2-5 yrs? Remember that a multifamily property will only appreciate a portion of the amount of that SFR projection.
Almost 20 yrs ago, I bought a $104K duplex for close to retail and lived in one side. The other side we rented out for $750, so the ratios are very close to what you are showing. Because the duplex was owned by a trust, we paid rent like any other tenant. We owned it four yrs, and put a roof and condenser units in both sides. When we sold, the only reason we made any profit was because we sold it ourselves at a market peak. The only money we made was capital gain. The rest of the spread went out for maintenance. And we had a personal mortgage, and we managed it ourselves.
You can save yourself a lot of trouble by drilling into the numbers in your analysis before you buy. Instead of spending your time getting locked into a project with little to no profit, spend it looking for assets that you can arbitrage for a lot higher yield. Look for doubles, triples and home runs. Start by hanging around others who demand yields commensurate with what value they put on their time.