How risky to buy very old house for duplex?

[John Hanson]

I'm looking at a duplex that was built in 1870, so it's about 150 years old!  However, if I assume monthly repairs of 1% of the value, plus capital expenditure reserve of 1% for a total of 2% of value, the Cash on Cash and Net Operating Income actually looks quite good.

I still need to learn specifics about what in the house has been replaced and what hasn't, but assuming that after inspection there are no major repairs needed, how risky is it to buy houses that old to rent out as a duplex?