This line is the problem:
@Html.DropDownListFor(model => model, new SelectList(Model.DDLOptions.CustomerOptions, "Value", "DisplayText"))
Your model is simply an int, based on the code above, but then you're calling new SelectList(Model.DDLOptions.CustomerOptions, "Value", "DisplayText")
in the partial too, referencing Model.DDLOptions, which does not exist in your model in the editor template. Your model is just an int.
There are several ways to do this, one of which is creating a custom model class for your item owner and have it contain the ownerID and the DDLOptions. Another would be to stick the DDLOptions in the ViewBag, but I usually stay away from that as I prefer using well-written, view-specific view models.
I hope this helps.