Did Sauron know about the Istari?

[The Saucepan Man]

There is not a shred of evidence that Saruman was ever Sauron's servant. Saruman wanted to compete with Sauron.

I agree. But Sauron was content to let Saruman have his way while it suited him (Sauron, that is). Saruman's manouverings around Isengard kept the Rohirrim occupied and helped make the gap of Rohan a treacherous passage for the "forces of good" (and would have done so for longer, but for Gandalf the White and the Ents). And that suited Sauron's purposes.

And that suggests to me that Sauron was not aware that Saruman was a Maia. Otherwise, he might have been a lot less happy about Saruman building up such a force and also (as noted previously) his attempts to get hold of the Ring.

Would a human wizard have lived as long in Middle Earth as Gandalf and Saruman? Would the fact that they lived so long not have alerted Sauron to their status?

I see no reason why a wizard, even a human one, might not have a considerably extended lifespan. So I don't think that this in itself would necessarily have alerted Sauron to their origins.