If the numbers have been verified, then grab it.

@Anita AhujaI think the rent would be closer to $1350-$1500, but you'd want to verify this with a PM company that is familiar with the area.Yes you can have the tenant pay the utilities.

The initial conversation he was not firm on the 600 and not opposed to splitting up the duplexes.

@Caleb HeimsothI've been doing this for 21 years.The Cap Rate on a Brooklyn, NYC property is not a fixed rate based on the initial purchase.These are moving Cap Rates.

Well again I prefer rents to sales price ratios when doing that quick initial math.

The outcome was a small profit but fell short of my initial goals.

I assume taxes are current if the local municipality hasn't taken the property back.If you can determine the amount of discount you'd get and add in the rehab cost to turn it into a SFH you can see if it's worth it.If you haven't already, verify that current zoning allows residential.

@Mathew Zorn- I think the others have already covered my initial thoughts on this, but at the risk of being repetitive, here's my take.I'd avoid partnering with someone who's currently in legal jeaporday at all costs. 

I absolutely realize that all that gain and my initial investment will have to come out of my IRA at some point and it will come out at Earned Income Rates, not Cap-Gain Rates.  

Apparently there is a big push going on now to allow ADU's in the city as a part of the Centennial NEXT initiative.