Can they prove how long it takes to rent a vacant property?

Find a better property manager.

Or local to the property?

Also, if this is something that you are looking for in a hard money lender, you might want to look into more of a private money lending model verse anything else. 

Any good area so you recommend for buying a rental property ? 

However, I could not find any listings on multi-unit residential properties on the platform.  

They usually say something like Pkg 5-dfw 28 llc, with 10-20 properties in each LLC.

Does anyone own rental properties in Detroit?

You won't open a "business" account unless your property is held within an entity.

We have couple turnkeys, and couple of BRRRs that we did so far (all are single family residential properties with long-term tenants).We are interested in exploring commercial real estate, and wanted to get all of your help in getting started.