Walk away.In order for the lender to obtain the necessary capital to fund that loan, it has to pass an initial environmental survey. 

If the it's on the walls, they'll make them remove the mold and patch the walls.If the owners refuse to do the environmental clean up, the underwriters won't approve it.

I also have ideas of building a town home or rental community but there is a lot of work needed with talking to the city, environmental study, financing, low income vs not, etc. 

The Zip code of 48221 is next of the list, here is the link https://detroitmi.gov/departments/buildings-safety-engineering-and-environmental-department/rental-property-informationI believe renovating and selling for top dollar would be in your best interest. https://www.clickondetroit.com/news/defenders/detroit-landlord-jailed-for-code-violations-including-apparent-black-mold-raw-sewage-at-home

On large projects like the one you are considering (a big one for "new" investor (your words not mine), we would have zoning, financing and environmental contingencies to mitigate the risk of closing and our earnest money.  

I'm guessing they'll look at sewer, parking, traffic, environmental maybe?

Call an environmental company. 

Usually that takes place because there might be a serious environmental problem. 

@Rob D.I keep recommending my clients have an- Attorney approval- Contractor review- Radon test- Lead based paint test- Asbestos test- Septic inspection- Finance contingency for FHA- $2500 repair expectation for any repairs FHA demands- $1 EMD- Engineers inspection- Furnace inspector- Partner approval- Environmental impact study at sellers expense- Neighbor interview- Plumber inspection- Assignment of contract- Well test- HOA rules approval- Certificate of Compliance acceptance- Marketability of title- Town code review- Survey- Soil testFor some reason they keep complaining they are not getting a response, beat out or end up spending thousands in tests and fees only to lose to some cash buyer when they request a repair.

Also check for any environmental issues: lead, radon, buried oil tanks, old gas tanks, asbestos, meth lab, etc...