How do you calculate rehab cost given you can’t look inside? 

So your capital gain when you sell this property will be calculate against your tax basis and not against your mortgage as many investors expect.

Do the calculations factor in default rates?

I suspect I can build this in 6 to 10 hours, but it is always helpful to have another brain on the job. 3) There is an advanced function I would like to build, but sadly I can't recall the mathematical functions necessary to make the calculations. 

I'm happy to chat about the calculator I use when evaluating properties along with my set inputs.

If the above is true, the income will offset the debt in the DTI calculation, so who cares? 

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).

Karl's Mortgage Calculator - This is a downloadable app for your phone. 

However, I’m not sure how to accurately determine its value, given the configuration and the fact that it’s not fully rented.My Concerns:Determining Value: How should I calculate the fair market value for this type of property?

I don't have the 10% for the down but thinking of using a HELOC to get the money and then add it into the calculator.