Help me analyze this deal

[Fred Hess]

I am looking into my first real estate investment property. It is an off-market turnkey rental property that already has a tenant placed. The tenant has been here for 18 months and is interested in signing a 1 year contract with new owner. The property is 1 half of a duplex built in 1920 (C property in a C neighborhood). 3 beds 1 bath with a finished attic being used as a 4th bedroom. For the area, the house is in pretty good condition. My main concerns is the unknown age of the roof (10+ years), original windows (roughly 12 total), and the 5 young children that will be living in the house. 

The seller is asking $100,000 for the property, and I listed $90,000 as the purchase price in my estimates. Please take a look at my numbers and let me know your thoughts on the deal. Thanks!