This has all the makings of a morality story.
The dragon in question would clearly be either an Oriental dragon (Ch'iang Lung or Tien Lung) or a Gold dragon.
The dragon's goal is partly to be repaid, but also to teach the wastrel king a lesson in fiscal and political responsibility.
The king in question should probably be noted for being "aggressive" in his dealings with his neighbors, and for having "too large" an army and spending "too much" for luxury items he doesn't need. Not necessarily to the point of covering his palace in gold, but certainly more than he needs to.
He can be relatively intelligent, but too given to listening to people who want to leech off him, or surrounded by sycophants, but no matter what motivation you give him, he should be honestly concerned about and care about his daughter.
Otherwise, there's a good chance that the party will just blow off the quest, if you make him too unlikeable, too arrogant, or too foolish.