With regard to how you should address this letter, if you know the name of the person you are writing to, you should use that ("Dear Mr Smith"). If you do not know the name, then use "Dear Sir/Madam". If you know perhaps the job title you could use "FAO Senior Tender Officer" (or whatever the job title is). "FAO" = For the attention of.
I'm not sure what you meant by "multiple choices" but that made it sound like a test so I have changed it to "multiple options". I assume you mean a variety of memory sizes or options of having different disk drives on the 25 desktop computers. However, you might want to clarify what you mean in the letter.

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