Age: Is irrelevant, I've seen more than my fair share of 18+ corrupt GMs
Name: Is irrelevant, ?
Experience: Is irrelevant, meaning you can train them anyway
Gender: Is irrelevant, because a lot of guys pretending to be female GMs and have really made real female GMs look bad.
Ultimately it comes down to trial and error, the more power they get, the higher the chance of them doing something corrupt. There is no true way of hiring a GM and knowing for certain they wont be corrupt. Also, always have a HUGE list of dos and donts, without this list... the chance of corrupt activity increases greatly. Personal Character, are they stable or emo? If they are emo, don't hire, if they are a known troll DONT HIRE. If they are brown nosing, DONT HIRE. If they aren't contributing or trying to contribute to the server, DONT HIRE. Donations do not count as a contribution, you shouldn't let people buy their way into power.
Legit Chars: Is irrelevant, a corrupt GM would find a way to bypass this rule anyway. I've been on servers where admins broke this rule that they themselves created. It is a pointless and bias rule, that only drags your GMs to other servers, thus making them less active, thus making them less in tune with the community. Also decreases the chance of them finding bots and abusers.
Head GM: Hire a GM that will control all the other GMs, don't waste your admin time trying to play GM... Admins and Developers make horrible GMs and they should never be involved in such matters, EVER. I can't make this anymore clearer than I can. Keep admins out of GM activities. If I had a dime for everytime an Admin interfered with me banning a troll, I'd be a millionaire.