Hi - unless it's your ancestor whose birth cert you have a special interest in, try to get the birth cert for the youngest child - as recent as possible is usually best for information. And ask for the photocopy, not the transcript (photocopy is cheaper, anyway).

No, I'm not in the IOM - in Australia. I suggest joining the IOM FHS if you haven't already, because they will be very helpful during your trip there, and will open their library especially for you if you are there on a non-opening day(s).
See: www.isleofman.com/heritage/genealogy/fhs/application.aspx
I'd join well ahead of your trip to give yourself plenty of time to get tips from them beforehand, and to see their journal with contacts and other information.

Sue