I currently own a 4 unit using a similar strategy. Depending on the year the property was built as well as the state of the property , i would plan to spend a few thousand on repairs when you buy it (might not be needed).
I would make sure to plan/budget/set aside money for management and capital expenditures. you might not hire management yet but it would be nice to include it in the budget in case you ever do.
I'm not sure about NJ, but in Allentown, Pa you also need to register as a landlord as well as register each unit with the city.
Using my numbers... Id assume 150 maintenance (6% ish), 100 management (50/unit), 216 vacancy, 150 capital (just a guess), 15 registration, 75 water/trash/sewer (your numbers) so about 705$/mo in expenses. Granted you would be keeping the 100 by managing yourself. so 605 in expenses every month on average. That still leaves you with 400$/mo cash flow or 200/mo/door which i think is pretty good from what ive seen!
I only own this one property and Ive only had it for less than a year so I would defer to more experienced advice, but there are the numbers the way i started analyzing mine. hope it helps!