It's all going to depend on what it says in your Managment Company's articles. If you don't have a copy, you can get them from the CRO website for a small fee.
In my personal experience, your development will only benefit from having residents involved. I'd be astonished if the other director refused residents as directors.
The agenda for the AGM is usually sent out with notification of it. You should put something like "re/election of directors".
Once a quorum votes on something at the AGM, unless it says otherwise in your articles, you shouldn't require the other director's approval.
"Will any resident who goes as director have the same amount of power as the other directors if that director still owns unsold apartments in the development (still in name of developer)?"
This, and votes for not fully paid up members, will depend on how voting at the AGM is laid out in the articles.
Best of luck!