I have never heard the multiples of 18 before, but money is definitely the appropriate gift for a bar or bat mitzvah.
Generally, instead of saying "congratulations", we say "mazel tov". It pretty much means the same thing. The translation of tov is good. I'm not sure about mazel. We say it for weddings, bar mitzvahs, new babies, etc.
Of course, reaching bar or bat mitzvah age, basically means that the child has reached the age of being considered a man or woman in the eyess of the religion. When a prayer session is to be held, a minion is needed. This consists of a certain number of men. (I think it's 12.) (Women are not deemed particularly important in this respect.) Anyway, a boy has had his bar mitzvah can now be considered part of the minion.
One more little bit of info: a mitzvah is a good deed. For example, giving to charity is a mitzvah.
So, I think mazel tov will suffice on the inside.