Evaluating underperforming multifamily

[Cameron Hayes]

I am interested in a multifamily property, but I have no clue what it's worth. There are 3 buildings on site....1st building was built in the 1980s and has 8 units, 2nd building was built in the 2000s and has 8 units, and the 3rd "building" is a concrete slab, prepared for 8 more units. All units have been neglected by property management and need rehabbed. Obviously, the 3rd building needs to be constructed. Out of the 16 rentable units, only 7 of them are rented at $500/month. There are no records of expenses or any other way to determine the NOI. Once the property is rehabbed, the 3rd building is constructed, and occupancy increases to 95%, the gross rents will be $136,800/year.

So...how can I determine the value on this?