Oooh, King Dedede! That should be fun! There are plenty of pictures all over this site:
I think you should definitely make him the coat, hat and belt. Since Dedede's feathers are blue and tan, he could wear a shirt and pants with some combination of those colors.
As for the beak... I'd make one like those sported by the Rito in Wind Waker
. Just get a triangular piece of yellow cardboard, fold it in half over his nose, and use elastic to keep it there (kind of like wearing a party hat over your face).
You could paint his face blue, or you could get him a blue wig... or both.