Yes what eschwartz said is correct, you can use Calibre to convert your existing epub books quite easily.
I love the kobo readers, though, so that's what I would get
Also, the Glo and the Aura are both excellent, and the sale price on the Aura is good. I wouldn't buy the official case for either one, I use other brands. As long as the used Glo is not damaged, should be good.