1- The wizards were maia, lesser spirits embodied as men and sent to fight Sauron. Of note, Sauron was also a maia. And there is a school of thought that says Tom Bombadil was as well.

2- The maia, though lesser spirits then the Valar, were, for all intents and purposes immortal. Thus Gandalf the Grey was reborn Gandalf the White, though he was esentially the same individual. In keeping with this Sauron was never destroyed or killed utterly, but instead remained a spirit, though his final demise was rather dark.

3- AFAIK none of the wizards adopted or had biological offspring. Though I am not the loremaster that some are on this board, and so, as always, I will bow to their superior knowledge if I am incorrect in this matter.

-RR