Yes, Intel Mac needs Kontakt. Kontakt and Kompakt, initially don't have many differences, if I may say so. Of course Kontakt can be hugely more complicated, since it's a sampler and not a player, but at the basic level it's a little bit the same.
You should google "midi" and learn a little bit before buying anything however. As for Sibelius, I'm not 100% sure that it will work well with EWQLSO. Maybe you should wait for Sibelius users to chime in on this.
