As mentioned no need to hire a private investigator just use an online service to do a background check.

Like you mentioned and what I've read is the deal structure and sponsor carries the biggest risk so thank you!  

Not to mention he is financially quite well off with or without my project. 

As others have mentioned, you can purchase a MF property as your new primary via Conventional with only 5% down without having to refi your current property.

Like others have mentioned Detroit, just like most any large cities, can be very street by street especially if you're chasing cash flow.

Otherwise, the other points mentioned are valid.

this property is relatively newer (2003) and I wonder what can be known about the two items you mention?

As others have mentioned above, the first lien might have a large payoff leaving you with little to no equity. 

From your past posts, as well as what work you've mentioned being comfortable taking on yourself, I think you definitely have the requisite knowledge to act as your own GC and sub out any work you don't feel like taking on yourself. 

Like the above poster mentioned, the issue is probably loan type more than the tax agreement.