You definitely want Arachna, she should be owning arena around your level, and she'll continue to be relevant, if not so incredibly dominant as she is at your level range. Skip Ember, Boomer, Shirley, Crusher, and Esabel if you're asking my opinion for 5 most important to avoid. Cara is a good investment for arena. So are Dokras and Garrick. They should be solid for a good long while.
I've heard some people say Shirley is actually pretty good at end game if used correctly. I don't have her though so can't really say much.