As I do things like this myself I really have no idea how much it would cost, I would think less than $100 if that much. Since you’re replacing the coolant adding more would be a waste of money; you also need to use MB coolant. Use tap water for now, it will keep the engine cool and not harm anything until you get it to a shop. Just ask for cooling system service which should include a flush, pressure and leak test, and a refill of the proper coolant at the correct mix.
Your 300 SE is a 126 chassis car, posting in that forum will get you more responses as that is where other 126 owners post and read.