The repair was extensive and costly, requiring excavation (about 3x6 feet) to replace the broken section and clean up the mess.

The high end is the 20 or so co ops on 5th Avenue or Park Ave (one is on East End Avenue) that require liquid assets of $100 million + to gain board approval.  

The biggest thing with 5+ units is everything is more expensive, and the inspection requirements (depending on the municipality) are often more rigorous.

Very subjective question based on your investment knowledge, home market, investment strategy and requirements. 

Coupling all of these (if capable) will grant the most favorable return.I know some buyers will not be comfortable buying a multi unit and some aren't comfortable with buying a home that requires renovation, and some won't qualify for down payment assistance.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Unfortunately NY requires a RE license to wholesale in the state.

The standard rule is $250k per person who legally owns/occupies for the 2/5 year requirement. 

I'm fairly new in the industry as well, and most of the sources I've checked at first require a subscription (CoStar, WSJ), but I found this newsletter called Zeroflux Real Estate who consolidate real estate news from such sites.

You would arguably be better off buying a CD at a slightly lower yield with almost no risk, time, or stress requirements.