I don't know much about historic districts, unfortunately, but you asked for any things to watch for: Lead has already been discussed here and it is unlikely to be a problem if there is no flaking or rubbing areas on the paint. You are buying a duplex, so you will have a tenant who might not be as concerned about it as you. They could cause some issues if they try to do some modifications. You should get a test done in your diligence.
Have your inspector pay close attention to the wiring. Those old houses will have knob and tube if not updated. Knob and tube usually works just fine but future buyers can be scared off and insurance companies sometimes get queasy about it. While they are looking around in the attic have them look for evidence of bats.
Basements like to flood. If you have a basement, make sure the inspector looks for evidence of water. This time of year, it'll be dry as a bone down there, but in the spring it can be a lake.
Some of those old houses will have remodels where they put in vermiculite insulation in the walls. This contains asbestos. As long as it is in the walls it is not a problem for health but once you do remodels it will be a major cost. Find out now if it is present. It will take putting a hole in the wall(s) to find out so make sure that the seller is comfortable with it. Don't just inspect the attic, vermiculite may not be there. Another place that they put asbestos is in floor tiles. Same deal here: as long as you don't disturb them its probably not a problem. Hopefully you have all wood floors in your prospective property and don't have to worry about this.
If this is a place you intend to live long-term these issues are not as acute. Since it is a duplex, you may have tenants that are concerned about it. Find out now to avoid problems later. Good luck. Those old houses can be beautiful and more solid than the newer ones if they are well cared for.
