Level is 65 to unlock so that's pretty much when you can start doing it. For power, I would say anywhere higher than 14-15k should be good. :) I've seen 12ks do it. It's really about the strategy. Of course more power helps :D
don't think this info is correct. it's true you unlock "Difficult" mode when you hit 65, but Easy/Medium difficulty should very well be open before 65.