I dont think there can be a onesize fits all criteria. The idea of a sub-forum to discuss not just moderators but sight issues in general isnt a bad idea. It would e like a sounding board for ideas or trouble.
As for who should become mods, I think that it shouldnt really be based too heavily on just the number of posts but the quality and also has the right temperment. Basically they need to have the right personality, inclusive and supportive. In my mind it needs to go beyond just having X number of posts, even if they are valuable they may not really be right to moderate.
Moderator isnt exactly a huge deal anyways, I dont really exercise my powers very often other than moving threads on occasion. People already do get new titles. If people feel the need to be recognized you could always add yet another color to the names and people with 1000+ posts or something would have that color.