Off market duplex, fair price?

[Emillio Henry]

I found an off market property that is significantly aged “1877” brick colonial duplex, with “carriage house” in rear. Homes in the area are going for $130-140 sqft. this property is 1400sqft.
2- 1bed room 1bath units.

I’ll be walking the property with the home owner tomorrow and I’ve already thrown out a $200k offer depending on condition. Owner believes that’s a low ball offer however that’s actually a great market value based off of the the comps. The home owner is a architectural engineer and im hoping he has applied his skills to the up keep of the property. 

My question is how high above market value would you go for an investment? I’ve run the numbers using multiple scenarios and I can easy get a 7% cap at $250k however I don’t want to over pay out of eagerness. 

Thank you all.