Your not getting the selected value because your not setting any selected value. Also, your view is confusing, your looping over the drop down list in your view, you should be doing that in your model or controller (preferably model)....but not i guess its to show 1 type drop down per customer? Anywho...
To set the selected value on SelectList's you need to do something like this.
List<string> dropdown_content = new List<string>();
var query = from m2 in db.custtypes where m2.active == true orderby m2.name select m2.name;
SelectList list = new SelectList(query);
var selectedItem = list.FirstOrDefault(m=>m.Value == id);
if(selectedItem != null)
{
selectedItem.Selected = true;
}
ViewBag.customertype = list;