Depends on what you mean by rare I guess, under 500? Under 1000? Just highly desirable? Get a machine that you will enjoy regardless of its rarity.
The King Kong pinball machine in particular is super rare, only 9 were made before they canned it. There was someone offering $25,000 for one here a while ago with no takers.
Get a machine that everyone will enjoy and fits into your room. A Star Wars, Dialed In, or Ghostbusters machine will cost a fraction of King Kong and provide years of enjoyment.
Welcome to Pinside!
-Matt