Which moderator levels should be available for users ? (gain using the reputation / privileges system)

None. Moderators are just a flat out bad idea. They could turn into trolls and just unpublish and trash around good content. There is really no need to edit someone elses work to make it not theirs anymore because a moderater edited it too much.

Do you need more moderators? Or is this just a plain question? I'd suggest that if you did need any, perhaps create an application for users who may be interested in becoming one, and select the ones who are: On the site, Trustful, Polite, and follow the rules. :)
This is just a suggestion though, and if any of it sounded offensive (I honestly don't know if any See More of what I've just written is...) I apologize.
But if it is for users, I don't believe they should have any. Like many others had said below, it could create quite a problem.
I hope this helps! :) And best wishes!

None. They could change other's work. You say those abilities would be taken away if they were abused, but then what's the point? To add an alternative ending to another user's story?

All but only for trustworthy users who would use it for good.

None. It's not right.

None. If users could gain moderator powers, it'd be easily abused. No one should have the power to edit others' works.

Qfeasters that have been on here for a long time should have these.

I'm not sure if it's a good idea to use the privileges feature to select moderators.

It would actually be cool to have some Moderator powers. Just not too many or we're gonna get mayhem for sure. But I agree with @Nephrite that only the trusted users should. Maybe you should make a system that highlights the trusted users when they reach a certain goal?