His suggestion was to wholesale it because he believed that for the purchase price of $90K, it'd be a relatively safe bet with a good margin.So, my question is: How do I get cash buyers or "buyers lists"?

I would check with a few different groups to make sure that they can operate in Detroit, and also to make sure that they will lend to the property in question based on the value.

There are a few DSCR lenders that will allow you to operate a rent by the room strategy as long as the strategy makes sense, and no one related to you is one of the tenants.

I'm an experienced workforce housing owner and operator.  

Or ask the third owner if they would also consider selling, so the whole property is sold.Whether a partnership or LLC, follow the Operating agreement. 

For example, if you wanted to outsource that part of your business, are there management companies in your area that you would feel comfortable turning operations over to?

Focusing on the foundational basics and your operations is more important long term.   

Your best bet is to invest in growing areas that are landlord friendly and where you can either connect with or build a good team yourself- realtor, property managment, contractors, etc. 

I operate several mid term rentals and I'm still a big fan. 

The best bet is to talk to a small local bank (or three) about your options.