my first duplex - is this a good deal?

[Dan Guerrant]

Been talking with an elderly private seller who wants to sell a brick duplex, 2br/1ba. 1540 sq ft. Good location, great shape, HVAC about 15 yrs old, 5 yr roof.  He seemed to think it was worth around 130K based on his improvements, but I'm starting my analysis based on the County taxable market value of $88,850.  He mentioned seller financing, which would be great since I very little to put down, although I believe he still has a mortgage on the property.  Here is what I have come up with:

Gross Mo. Rents 1100 (550/unit)
Prop Tax               150
Insurance                40
PM 10%                110
Vac 10%               110
maintenance 5%     55
CapX 5%                 55
Total Mo. Op. Ex.  520
Mortgage               424  (I'm new to seller financing, so I've started with a  30yr / 4% / 0 down estimate)
Monthly Profit        156

NOI 6,960
est price            88,850  (based on county taxable market value)
Cap Rate             7.8 %