How do I run the numbers for a triplex?

[Raleigh Lewis]

I have what i think is a "great deal." It is a triplex with 2bd 1 bth 1st fl, 2bd 1bth on 2nd fl, 1bd 2bth on 3rd floor. There aren't any triplex near it within 20mi which is a few counties over. I have researched the rents in that market and i found out that i can charge around $815 for the two 2bd rooms and $771 for the 1bd. The property needs to be rehabbed I estimated the repairs around 40k and they are asking 50k it is vacant but i see much potential. The property is in a rougher neighborhood but is basically on campus. The property is in Chester, PA. How should i run these numbers? Should i look for a NOI or ARV?