Your monthly mtg payment includes principle and interest, the principle reducing your loan balance.  

I have no experience with financing larger apartment buildings and I’m wondering about down payments.

As a recent graduate there be programs in MI that will help with your down payment, there may also be programs in Detroit that can help as well if you live within the City limits.

That's why you have $9 monthly payment  

In Michigan, you HAVE to provide at more than one means of rent payment.

The 50% rule is related to expenses, which does not include debt service, so your down payment won't affect that.If you're not planning to live in the property, you will have to put down at least 20-25%.

And my wife for the support.The best part of it all is we just found out we’re having a baby girl early next year and we added $100 automated principal payment each month to our 30yr fixed mortgage so that it amortizes when she turns 18 as @brandonturner suggests.I’m so grateful for bIggerpockets and the community it’s formed to inspire and support me through this journey.Next year I hope to do my first BRRR!

Originally we borrowed $240k from my Grandma and then repaid her (with some interest payments) 9 months later by doing a refi.

After expenses, which included a special HOA assessment and a month's rent to my agent, 24 payments of $1595 ($38k) left ~$12k in rental profit.

I am doing a Promissory Note with 0% interest, zero payments for the same length of time of the loan the buyer is taking out.