I'm no admin so you should maybe take my response with a grain of salt, but I think that they would need to be quite extraordinary when it comes to their education, because it's kind of a serious job. For example, Aurors are required to have five N.E.W.T.s of at least 'Exceeds Expectations' as the minimum grade. If I were you, I'd shoot for something around that range, while still being realistic. I would imagine she'd need to have the focus and drive of a very good student, would need to take great care and study for all of her classes, and would need to put her education as a top priority. Unspeakables deal with things of a serious nature, somewhat dangerous in many cases, and need to display trustworthiness, perhaps by an assessment or by being handpicked from another department? Although there is, of course, the possibility of employees making an "Unbreakable Vow" with the Department not to tell the nature of the job? Not really sure. Just my thoughts. Maybe an admin can clear more of this up for you later, but long story short, I'd suggest having top-level grades.