According to standard etiquette, it's best not to even mention that the book is part of a planned series. That can come up after the agent's shown interest, and at that point it'd probably be a nice plus if you could say that you've got the rough draft of the second done.
Multiple submissions usually means that you're submitting the work to many agents at once. It's polite to note that somewhere in your query letter. Of course, if they're the only agent you're submitting to, it's nice to note that as well.
Yes, it's absolutely recommended that you tailor your query letter to specific agencies. This is a pain in the ass. You'll generally find they fall into one of a few categories though, and it might be possible to have Query Letter A, B, and C sitting on your hard drive. Sometimes it's just a matter of formatting. Do they want the blurb first and then the book info, or info first and then the blurb? Do they want your address at the top or the bottom? Cut and pasting will solve many problems, I've found.
Nathan Bransford has a
section on his forums specifically for posting query letters and getting advice. Edit out your personal info and try posting it there. The only coin you need is to post a few reviews yourself of other peoples' letters. Just reading the advice people give can be quite the eye-opener. In the end, you'll still have to tailor your query to specific agents and there are a few somewhat unhelpful people who critique based on one particular agent's style (they'll insist you always start with the bio, for instance, when some agents prefer to start with the blurb) but all in all it's helpful.