If you know you want a king sized bed but are hesitating about picking one, consider first investing in just a metal frame with wheels for your king sized box spring and mattress. You can go ahead and use that and wait about choosing a headboard. You may well be able to find a metal frame that is adjustable and can be either a queen size or a king size. If so, choose that.

I really want headboard & footboard

Then it's time to go shopping!

I have no idea what you want/