Hi Greg,
Welcome to the forum, I think this helmet is a nice authentic Haube, and it has the added feature of being a rare large size. Most helmets were made for children, not as toys, but to wear in combat. If anyone tells you that you paid too much for it, you need respond that you did not pay too much, you just bought it too early (I think this is high for an M15, but I got mine for $19.50 so don't pay any attention to me). You should not have any trouble selling it for the same or more, so to answer your first question, "no" you did not get ripped. I have not searched for Kockarden or chin straps recently, but they are out there, and hopefully another member will be able to answer that question.
Best wihses
Gus