I think Sauron must have known of the Istari, both becuase of their long lifesapn and because of his alliance with Saruman through the palantir.
A note on human wizards from Tolien's Letters #173:
"Beorn is dead... He appeared in The Hobbit. It was then the year Third Age 2940... We are now in the years 3018-3019... Though a skin changer and no doubt a bit of a magician, Beorn was a man."
This seems to imply that even men with magical powers would die in a normal lifespan. I think it is also important that he used the word "magician" instead of "wizard". (Just as he used witch instead of wizard for the Nazgul)
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I don't feel safe in Middle Earth no more... I don't want to die in the ruling ring war... I want to sail away to a distant shore... and make like an ELF man
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