Your top level math is pretty accurate but is very hypothetical.
Getting just over 1% of purchase price in rents will cash flow some (hypothetically with 50% rule) but will be a pretty lousy CoC.
Generally on a 2 - 4 unit building you'll have to put down 25% (20% would be for single family homes).
For interest, you can get conventional NOO in the 4.25 - 4.75% range today dependent on your credit.
For people that are looking for $100/rental or door... they're looking for $200 of cash flow out of a duplex (2 family, 2 doors, 2 * $100)... it's not a per bedroom calculation.
Things to always watch for are taxes (I like them in the 5 - 10% of GOI range, lower is obviously better)... insurance (floor insurance can be a killer)... and utilities (many landlords pay water though few are happy about it... you really don't want to be paying gas and/or electric as that will blow 50% out of the water). You can also look into vacancy rates and Property Management costs in your area. They will vary some.