Sauron and Istari

[Sarumian]

I wonder how much Sauron was able to find out about Istari, their origin, nature and abilities?

[Inziladun]

Anything he knew in detail, I think it likely he would have learned from Saruman when the latter began to use the Palantír of Orthanc. If Saruman did not directly tell Sauron, he could have possibly deduced some things he wanted to know anyway, owing to his domination of Saruman's will.
After all, the true nature of the Istari was only known to a select few in Middle-earth, and they were mostly in Rivendell, inaccessible for capture.

Still, Sauron may not have known the entire truth, but it seems he knew or suspected that Gandalf was the prime mover against him, and that he was not a mere Man or Elf. The Mouth of Sauron's words to Gandalf are telling.

'Then thou art the spokesman, old greybeard? Have we not heard of thee at whiles, and of thy wanderings, ever hatching plots and mischief at a safe distance? But this time thou hast stuck out thy nose too far, Master Gandalf; and thou shalt see what comes to him who sets his foolish webs before the feet of Sauron the Great.'

ROTK The Black Gate Opens

Also, Sauron in his Necromancer guise apparently encountered Gandalf personally before. In The Tale of Years, Gandalf is said to have first gone to Dol Guldur to investigate the power there in the year 2063, and Sauron "retreats and hides in the East". Maybe he felt something of Gandalf's spirit then, since both were in origin the same order of being.

[Zigûr]

In "Notes on Motives in the Silmarillion" in the "Myths Transformed" section of Morgoth's Ring Professor Tolkien observes the following regarding Sauron:

"If he thought about the Istari, especially Saruman and Gandalf, he imagined them as emissaries from the Valar, seeking to establish their lost power again and 'colonize' Middle-earth, as a mere effort of defeated imperialists (without knowledge or sanction of Eru). His cynicism, which (sincerely) regarded the motives of Manwë as precisely the same as his own, seemed fully justified in Saruman. Gandalf he did not understand. But certainly he had already become evil, and therefore stupid, enough to imagine that his different behaviour was due simply to weaker intelligence and lack of firm masterful purpose. He was only a rather cleverer Radagast - cleverer, because it was more profitable (more productive of power) to become absorbed in the study of people than of animals."

I would argue from this that Sauron must have been aware of the Wizards for some time, but was not especially threatened by them, any more than he was by the Lords of the Eldar, at least, and was at least in some regard ignorant of their nature and purpose. I daresay this was part of the plan - not only did their humble shape make them readier counsellors rather than leaders for Men, but eluded the full attention of Sauron, who surely underestimated them, and Gandalf in particular.

[Boromir88]

I agree with Inzil that probably whatever Sauron knew came from Saruman and also Sauron's reasoning that only sees others in his own skewed mind for power. Like as Gandalf tells the Council:

...the only measure that he knows is desire, desire for power and so he judges all hearts~The Council of Elrond

Tolkien wrote that Sauron was not "sincere" atheist like Morgoth became, that is he never denied the existance of Eru and the Valar...his plans to dominate Middle-earth just came about from no longer fearing Eru or the Valar getting involved in Middle-earth's problems. In HOME X: Morgoth's Ring there is more explanation that is related to how Sauron viewed the Istari:

He probably deluded himself with the notion that the Valar, (including Melkor) having failed, Eru had simply abandoned Ea, or at any rate Arda, and would not concern himself with it any more. It would appear that he interpreted 'the change of the world' at the Downfall of Numenor when Aman was removed from the physical world in this sense: Valar (and Elves) were removed from effective control, and Men under God's curse and wrath. If he thought about the Istari especially Saruman and Gandalf, he imagined them as emissaries from the Valar, seeking to establish their lost power again and 'colonize' Middle-earth. as a mere effort of defeated imperialist (without knowledge or sanction of Eru).~Morgoths Ring - Myth's Transformed Text VII

This fits with what Gandalf judges about Sauron, he can only see people through his own power-tinted goggles. If he thought about the Istari, he just saw them as some rogue emissaries sent by one of the Valar to carve out a certain sphere of influence in Middle-earth for themselves. He never appeared to think the Istari were actually sent on an Eru-sanctioned mission to bring about his defeat.

Saruman fell away from the mission to his own designs and lust for power, and this is where Tolkien gives as the reason that Sauron was always able to understand Saruman more, while Gandalf had always eluded him:

Sauron had, in fact, been very like Saruman, and so could understand him quickly and could guess what he would be likley to think and do, even without the aid of the palantiri or of any spies, whereas Gandalf elduded him~ibid

Sauron understood Saruman almost right away, because Saruman is essentially a Sauron-lite version. If Sauron drank whole milk, Saruman would be drinking skim. Gandalf was the one that eluded him, because Sauron was incapable of viewing the world without his lust-for-power goggles.

[Mithalwen]

Were not Saruman and Sauron both maia of Aule, originally which may have been a factor in their affinity- literally kindred spirits.

[Inziladun]

Originally Posted by Mithalwen

Were not Saruman and Sauron both maia of Aule, originally which may have been a factor in their affinity- literally kindred spirits.

Indeed. That may have been what led both to consider the use of the palantír at his disposal to further his own ends as well.

[Inziladun]

I think too that it's interesting to note a passage from the section The History of Galadriel and Celeborn from Unfinished Tales.
When first making contact with Celebrimbor and the Elves of Eregion in the Second Age, Sauron

posed as an emissary of the Valar, sent by them to Middle-earth ("thus anticipating the Istari") or ordered by them to remain there to give aid to the Elves.

Footnote 7 there goes on to add:

When [Sauron] came among the Noldor he adopted a specious fair form (a kind of simulated anticipation of the later Istari), and a fair name: Artano "high-smith," or Aulendil, meaning one who is devoted to the service of the Vala Aulë.

If indeed Sauron considered even then that the Valar had not forgotten Middle-earth, and might send someone to oppose him, it's notable that he chose for himself an attractive form rather than one appearing weak and old, supporting what Zigûr said about Sauron's underestimation of the Istari.