As Book!LotR fans are likely aware, Saruman was the same sort of being as Gandalf (i.e., a Istar and almost certainly a Maia) who was sent to Middle-earth with the same mission Mithrandir: opposing Sauron, not through direct power but by wisdom and inspiration of Men and Elves. Like Gandalf, Saruman presumably began his career in the Mortal Lands as a genuinely good being, though of course deprived of much of his angelic knowledge and power. He did not create build or even design Orthanc himself; that tower was built by the men of Westernesse, and Saruman was given leave to dwell there by a Steward of Gondor whose name I am too lazy too look up. It is said that Saruman petitioned leave to dwell in Isengard because he knew a palantír was there (presumably forgotten by Gondor) and that in attempting to use it for his own purposes, he fell under Sauron’s influence and became corrupt.
Which brings me to the thread question. Suppose the seeing-stone hadn’t been there? It could easily have been taken by one of the Gondorians when the abandoned the tower, or stolen by a Dunlending, or found by a passing Orc or Nazgul during the time of its vacancy and taken to the Dark Lord. The specifics don’t matter, at least not to me, except as it leads to the thead question: without the palantír, would Saruman have fallen?