Find an option for a return upstairs and dont use that contractor to do your work. Any contractor who says that is a hack. You will roast in the summer if you dont put a return upstairs. Returns upstairs are important. If possible put one in every bedroom. The reason for this is in case you sleep with the doors closed or close the doors at all for anything. If you cant put one in every bedroom put one in the hallway. If you can put a supply there you can put a return there. What size return will depend on the size of the upstairs.