Trixiee--I'm not a duathlete or a triathlete, but since no one else has responded to your questions, I've got some suggestions.
First, educate yourself--which I realize is what you were trying to do by posting here. But there's another way. I recommend two useful books: A Woman's Guide to Cycling and The Female Cyclist. The first is packed with good info on bike buying and fitting along with lots of other stuff for a new rider. The second book is a little more advanced, so may be useful for your duathlon training.
As far as choosing a saddle, most shops will let you try one for a ride or two, then exchange it for another until you find the one that's right for you. Ask before you buy.
I hope some of this is helpful. Most of all, have fun cycling!
--J
"Do I contradict myself? Very well, then, I contradict myself." --Walt Whitman