newbie trying to work the numbers on potential duplex

[David Keller]

I have a potential property listed in Statesville, NC for $160K but realtor says $140. It a  brick 1 story duplex with 1 tenant in place and they pay $500/mo. the second apt vacant but typically rents for $600/mo. I have to pay h20/sewer which $10-20/mo and the property tax is $1100/yr. HVAC new in 2013 and just serviced last week. Front porch replaced last year. The location is not the best but def not the worse by any means. 

Im assuming vacancy is about 8% and maintenance/repairs another 5%. Again being a newbie and trying to work in all the calculating formulas available I wanted to gather everyone insight and opinion on the property. What if any additional info do I need to ensure im figuring into this to ensure im getting a good deal.

Thanks gain for your time in advance.

David Keller