A duplex in Westchester is desirable. Many folks love the schools and the location to LAX. I checked the listing. It looks like the bank has been trying to sell it for over a year. The started around 900k. The units are on the small size. I haven't seen this duplex but it seems like a great deal. Without seeing it I would say 1600-1800 would be safe. The rental market is still strong. Especially if they are clean units.
Duplex's can be a good investment. It depends upon the duplex,income, location and your particular situation. I would recommend you consult with a Realtor in order to get the specific information you need.