1. Doesn't matter which server it is, donators will always have the advantage above an ordinary player. Yes, you can kill them but not in a 1v1 since they will outgear you and you will most likely lose. About general balance, I have yet to see the perfect server in terms of balance but I'd say this server is one of the most balanced out there. Only thing that makes this server a bit-much unbalanced is the bloody r8 and its random stats, if it was an r9 server it'd be more balanced and way more crowded.
2. I'd say seekers and mystics. Insane defense and decent attack.
3. This is a international server, it has players from around the world so it obviously also has indonesian players.
You are massively exagerating the first answer. I have no problem killing almost all donators with gear farmed trough pve. It just takes a little longer to get that way.
Seeker & mystics are great choices for class as you said, both are very strong all-round. Though archer might be a better choice for pure masspk.