The City Inspector does not add on more work...assuming you have the project correctly estimated in the first place.

He actually recommended that I go with an even lower price than I was initially willing to, to ensure I wouldn't lose money on my SBA loan estimate.

Estimated cost for a 1500 SF house $1,500 or more depending on your details.3.

See if you could shadow a flipper during a project to learn how that process goes, learn to estimate rehab costs (this is valuable even if you don't plan to ever flip!!)

I was thinking about an strategy of doing Cash Out Refinicing putting that money in the stock market (distributed in ETF and some "safer" stocks) to get around a 6% anual return (estimating that low because is the "standard" average, however my current stock portafolio has a cummulative return of 66% in the last 4 years).

Detroit's population is actually growing according to recent surveys not shrinking Census Bureau estimates: Detroit population rises after decades of decline, South dominates growth (michiganpublic.org)2. 

A good contractor can give you a best-case, worst-case estimate.

The interior and grounds need a major renovation, which I am estimating at around 1MIL.

Unless you are incredibly confident in your reno assumptions, I'd recommend getting a contractor out there ASAP to give you an accurate reno estimate if you want to go that path.

Not a day goes by when a vacation rental investor does not ask: "How accurate are AirDNA rentalizer estimates and projections for short term vacation rentals?"