The new windows will add additional value to the property, so consider it a $17K concession.

In addition, I think there's more "control" with stocks in the sense that you can pull your money out whenever versus real estate, the money is not liquid. 

In addition to ensuring that the room is built to the proper load bearing capacity as @Mike Reynolds mentioned, you should also keep in mind that you need a second egress point, such as an egress window, for any habitable room especially if it is designed for sleeping purposes.

Additionally, the strong job market and growing population point to continued demand for multi-family units.

Think about things like crime rates, employment growth, the caliber of the schools, and facilities while assessing neighborhoods.

I think you could leverage the equity to purchase additional real estate that may cash flow better, but it really comes down to what your goals are with investing.

I know that where I do business (mostly Arizona) we add additional clauses to our agreement for sale to help mitigate the seller being nefarious and providing enough public record to keep things on the up and up with title companies.

In addition I also helped him build a 100'x30' shed.

In addition to the platforms you listed, make sure you’re also listed on Furnished Finder 

We fell into the trap of giving them multiple chances and hoping for improvement, but it usually resulted in further delays, additional costs, and compromised work quality.