I don't really see how it destroyed his body, though one might surmise that his body couldn't handle 9 or 10 or whatever transformations since the first two alone jacked him up so much.

Also on the Kyuubi/Shukaku thing, that's a little different. They're both possessed by monsters but Naruto hasn't, at least not yet, had physical features of the Kyuubi (he controls his chakra but doesn't turn into the actual monster) whereas Gaara's is more of a transformation into the Shukaku and then is taken over by it.. and it commonly has a physical form, since it transforms monthly.