Don't get to excited,approach it as a numbers gameand make a decision that way.Have you checked the title and made sure he is the only one listed and verified his identity with a birth certificate or drivers liscense?
Have you checked the property for liens,judgements,unrecorded second,third mortgages,property tax liens and or irs tax liens,are the brownstones in a historical district?If so you might have to alter the renovation plans you had in mind or may have to get approval from the board on every renovation which could increase your carry costs and renovation costs.
Check the chain of title on the property to make sure there are not issues of ownership.
I would write offer subject to financing,clear title,satisfactory inspection,and no liens,judgements,back taxes,or enviromental problems with a backout period on your part.If you put anything down hold it in your brokers escrow account and do not give it to the seller!
good luck