I am also thinking of just converting to an ADU for a few years and once I have my savings built up going forward with ADU but that might end up being more expensive. 

always love seeing others getting into the RE game early in life. i used to be a die-hard index fund investor & have since moved all my $$ into RE. you just seriously can not compare the two. wealth can be built SO MUCH FASTER in RE vs in the stock market. would love to connect and see how i can help you!

My concern was small single person businesses are subject to robberies which occurred frequently in the area in the early 1980's.The building was built in 1890 and NYC established building codes in the 1920's, and someone owned it all the while passed and my dad offered to purchase it but was beaten to it by a flipper who got it for $23K, and he bought it from the flipper for $25K.

The house was built in the 1800s and although it is zoned as a multifamily, it has never been used as a multifamily before us.

This way you stay ahead of the market average of 7% and have built in value. 

What happens to your $150-175k if the family member who owns the land you built on needs to sell for some reason?

Roof, HVAC and foundation all have no issues.My plan with a property like this would be to get positive equity asap and re-fi.

I no longer work from a place of scarcity.I set myself up for success and work from a place of abundance.I have built my cash buyer list to close to 5,000 buyers, invested into a coaching program for private lending, and developed multiple exit strategies to be ready for any situation that presents itself.It let's me earn $40k per deal consistently today and occasionally hit some $90k+ homeruns.Embrace your mistakes, learn from them, and adapt.This is the only way to grow.

Will you be required to inject capital if they need it to repair roofs / foundations / plumbing / etc. 

That being said, it could be significantly cheaper to have a large finished shed built in lieu of a garage and it and can serve the same purposes for you.