Hi and welcome to the board.
I´ve actually just put my Traya up for sale... a bit sad about it. But that´s just life... http://www.theflagshipeclipse.com/forums/index.php?/topic/2187-fs-darth-traya-costume/
As to your questions:
You can use almost any dress pattern for a straight down one and give the front and back some more cm to it, so you can create the folds.
As to wigs - your link doesn´t work - but any silver one I guess. I found one on the net, just remember that with both the wimple, wig and headpiece, the wig shall not be to thick due to the heat.
The saber looks fine - though not all correct, but since we don´t need to get it approved with the costume, you should be fine.
As to the head bands, yes it is correct that they shall change in size from the top to the buttom.
For the belt I´ve got velcro, since the closure needs to be "hidden" in the back. I found it was the best way, I got mine custom made.
I´m looking forward to seeing your progress