I mainly want to ask if these assumptions are reasonable and if there's anything I haven't considered:(1) multifamily units in NJ close to New York City, ~$1,000,000, 20% down payment, (2) Using the following assumptions: 4% appreciation rate, 6.5% interest rate and 5.0% refinance after 5 years, $10,000 yearly maintenance fee(3) ~$6,000 monthly rental and assume 3% increase yearly with 5% vacancy rate(4) Based on the above, the calculated IRR if selling at the 10th year is ~19% (considering tax benefits) and ~17% (without tax benefits).

The sellers you are dealing with in these situations don't deal with prorating Taxes, or any other kind of prorations, that is what an escrow agent does. 

And of course we're in agreement on consulting a tax pro.

During escrow converted it to a Roth and paid the taxes.

They are so tax beneficial! 

. - Taxes are on the higher side at $6,000 yearMy Numbers: $115,000 putting 20% of my money $23,000 and finance the rest with total expense of $1,834Monthly expense numbers: Future Maintenance 13% $273 - Vacancy 5% $105 - Property Insurance 5% $105 - Property Taxes 23% $500 - Property management 10% $215 - Office/Travel/Legal 4% $84 - Mortgage 26% $552 - Monthly Cash Flow - $316 per month or $3,792 per year so Cash on Cash = 17%I think this looks like it is a deal worth doing and I also believe I can bump the total rent up by $50 each tenant which I think make it even better.

So you have this shadow plat underneath a 10 acre lot The county assessor since its all one ownership would just show the acreage and no individual lots IE not give them apn #s so if you were looking at the county assessor maps you would just see this 10 acre lot..

(I REALLY can’t stand state income taxes.  

You can use it as a downpayment on a property over $450k and use leverage tax free.

You could look into forming a C Corp as an alternative way to own this property that could affect your tax returns differently, I am not a CPA or giving tax advise.