Ooh, sounds like fun.
For invitations, you could make a 'treasure map' to your party location. You could hand draw it and then just run off copies, shouldn't be too expensive. You could also dip the paper in strong brewed tea to give them an extra old look (make sure the ink is waterproof before you try, though).
For the best dressed pirate, what about a custom eye patch with the date of the party on it? Then all the past winners can wear them to the future parties.