At the moment there is a lot of stuff in the shop that many people want to buy- all the sweet things that were one time only during the first year of GW2. So now that they are back many people want to buy them. They trade in their gold for gems and the price goes up. As soon as the special offers are over the price will drop again.
I would wait if I were you (as long as there isn't anything that you really want/ need).
Everytime there is something special in the shop the price for gems goes up, and then it goes down again.
Nope The launcher will download whatever is needed on launch day or whenever they allow you to start downloading the real game. It'll be a hefty download, of course, but it wont be the full 20GB+ you'd need to download otherwise.