With your current estimates, you're essentially assuming infinite lifespans for the property's various components (roof, foundation, HVAC, electrical, plumbing, water heater, etc.). 

I would bump your expenses to atleast 800/ month between 2 units of utilities and averaged maintenance and vacancy over the long haul as future gas prices, water charges, and maintenance could go higher. 

Any other expenses that landlord normally pays in duplex in your area like water/sewer/garbage, is snow and mow completed by tenants?

Another incident I had a tenant wanting me to pay her water bill after it was not repaired for two months.

The cabinets, flooring, and paint look to be in good shape from the photos, but what about the roof, foundation, HVAC, water heater, etc.? 

We have 1 or 2 properties that are not sub-metered for all utilities and 3 that arent sub-metered for water...

@Richie ThomasThinking through this after what you mentioned I think the owner is scared to sell it outright because he would be in hot water due to the condition and the tenants are likely not safe to what the county would expect.

Estimate.Tenants pay for water, electric and gas.Loan for full price 30 years at 4.125%

And the equally fatal opposite is jumping in without knowing how deep the water might be.

A new water heater?