I personally would look into running ducts to the second floor. without seeing the house I cant be sure but I would think it would be cheaper that adding a second unit but that is only if the current furnace is big enough. it would also be better down the road when you add air or switch to gas because you would only have one unit. but mlb is right electric base board would absolutely be the easiest and cheapest option but there would be no way to add ac in that case