you should not need any thing else in order to use the speedlight. It'll fit right on top of your camera. I would, however, suggest 2 sets of rechargable batteries for it. These things go through batteries pretty quickly and it BITES to get stuck w/ dead batteries.
I'm a Nikon girl, so I can't comment on the price for the Canon speedlight. Seems reasonable to me though.
can't comment on canon either and like gigi said unless you want to do off camera you don't need anything. One thing I'd suggest is reading the speedlight lessons on ILP (it's LONG). I still haven't gone through it all but the main thing I've learned is to set my speedlight to 1/4, 1/8 at least rather than full (not only will your batteries drain slower but it will be ready for the next picture much much quicker