Quick English -> French translation questions

I work for an advertising firm and was asked to translate a letter to clients (people who have businesses and pay for advertising space in our directories), and a couple of things have me stumped.

The greeting-
The English is:
Dear Valued Advertiser,
I first translated it as:
Cher annonceur estimé,
But, in French, is our client still considered "annonceur"? Or would it be better to put "Cher client estimé"?


And then, in the English version, it says "Please review the content of your proof(s)." When I translate that, can I put "Veuillez revoir le contenu de votre épreuve(s)", or should it be "Veuillez revoir le contenu de vo(s/tre) épreuve(s)", or should I just not worry about it and keep it singular?

Thank you very much in advance for your assistance!