Is there in the books or on Tolkien's letters a description of Sauron's armor during the first or third age, when he was not trying to "seduce" other races?
Is it accurately portrayed in the movies?
There's no description of Sauron's armour in the books - in fact, there's no indication that he even wore any.
The most full description of Sauron is in Letter 246, and reads as follows:
In the First Age, he wasn't an armoured giant but rather a sorceror, a wizard, a necromancer, even, and is described as such in the Silmarillion:
The depiction of him in the movies is an invention so treat it as fan-fiction. In general Tolkien himself doesn't go into this kind of specific detail with many of his characters.
There's an entry on Sauron's appearance at http://www.glyphweb.com/arda/faq/sauronshape.html with a lot of quotes from Tolkien's writing, seems like a well-researched site so I doubt they missed any detailed descriptions...from this, it seems Tolkien was never too specific in his description of what Sauron looked like, and said nothing about what he wore. But in support of his general appearance as a very tall humanlike being, there's this quote: "Sauron should be thought of as very terrible. The form that he took was that of a man of more than human stature, but not gigantic."