My biggest concern is managing additional units and tenants.

That's an ADDITIONAL capitol investment made. 

Hey Tim, If you are not using Hospitable yet that would be my first addition.

Removing it cost us an additional $5k over our estimate.  

We're fortunate to have an above-average income and live well below our means.This summer, we’ll be moving to Detroit (PCSing in August), and we’ve saved about $120,000 to invest in addition to having funds set aside for the home we’ll purchase in Detroit.The reason I’m posting is that I’m struggling to decide how to best allocate the $120k.

Give her 30 days notice now, keep her security deposit, and send her a additional bill for damages to the unit in lieu of taking her to court for the damages.  

In addition, there's a church, Bethel Church, and they have a ministry school, Bethel School of Supernatural MInistry, which is expanding to have 3000 students.

Additionally, as a new investor, I am curious if you would recommend this strategy based on your accumulated knowledge and expertise.

Additionally, involve a real estate attorney to ensure the transaction aligns with legal requirements and protects both parties’ interests.

This initial negative equity position consumes the initial cash flow.  2) the addition of the adu makes the property multi family and likely makes rent control apply.