Make sure you put the correct postage on it. You should not mention this. The reasons you would be the best person to write the article can include personal experiences, knowledge of the subjects you are writing about, or expertise in a field. No, those are works that speak for themselves. In these instances, you would include a short cover letter.
Books require a book proposal.
A simultaneous query means you are sending your idea to multiple magazines at the same time. Many editors will not accept a simultaneous query. Check the guidelines to find out which ones will. At the end of your query, write a brief sentence stating this is a simultaneous query. Do not try and hide the fact that you are querying simultaneously.