Let's be optimistic and say you will end up with 4K positive cash flow for a year, which is 10% cash on cash return. 

It seems over priced and using overly optimistic rentals to me, for the long term.  

Am I being overly optimistic?

I'm cautiously optimistic about the future of the property :)       

That tells me they were a little too optimistic with their listings.

@Jimmy SolanoI think your expense estimates are way too optimistic.  

I reckon 3% vacancy allowance is far too low, and, perhaps $2,600/m is too optimistic?

My realtor tells me this is no problem but it seems overly optimistic to me.

my price range is probably around the 350k range for a duplex... in Lynn/Lowell MA. ...So if I bought a duplex, and lived in one unit and rented the other for 1.2k, I would pay 1k a month in mortgage expense assuming my mortgage is 2.2k"...If total rents once you move out are (only) $2.2k/m, on a property that you paid $350k for (95% leveraged), your negative cash flow will be much more than the "$100/m" that you optimistically guessed earlier!

Optimistically, that means I'm looking at sometime in 2020 before I can get a loan.