I had a similar situation in which I bought a 20 unit at 100% occupancy and 90 days later had 8 vacancies, with NO placements at all.

I also estimate that, after accounting for a 10% vacancy, I could rent the property for about $1,800 per month.Even with that ($400) cash flow, I still feel like I’d be better off than where I am now, between the experience I’d gain, the equity I’d build, and the appreciation and tax benefits.

15% vacancy rate is high.  

I know there are the obvious PITIA items but what about property management fees (I plan on using a property manager), maintenance/repair allowance, and vacancy allowance?

Their pro forma sheet doesn't take into account ANY vacancy or repairs when calculating cash flow, and the rent comps are...ambitious. 1.

Commercial property can take a considerably longer to fill a vacancy.  

It's highly unlikely you can get anything other than commercial insurance now but even that could be challenging based on how you're using the property and the commercial vacancy

Ideally, along with your mortgage payment, you’d want to set aside funds on a monthly basis for repairs, vacancy & capex if applicable.

Rougher parts of town have increased maintenance/cap ex, vacancy, rent delinquencies and defaults, and level of effort to manage.  

I’ve used furnished finder and airbnb to see listings so I can get an idea of rates but vacancy has been a little more difficult to nail down.