You really want to think of this first. Yes, you can absolutely make 25 mages, and no player will live if you push the AoE button for all 25. Boss monsters in dungeons or outdoor raid bosses will live though. I see three things that you can do with a 25 man group:
a) PvP+PvE attack the cities of the opposing faction. (There is no character limit as this is open world movement.) That should be the best option, because in a nutshell you are the event then. =]
b) PvP+PvE 40 man battleground Altherac Valley. (In the normal game client, one can sign up to AV only with a party of 5, but there is a Horde only addon that syncronizes up to 8 parties for a full 40 man raid group.) There are a number of strategic mini objectives in different places on the map that you will not be able to fulfill. On the other hand, you can arrange for 15 other people to group with you and sort those. Nuking the miniboss and then the boss of the opposing faction is definitely possible. If you do not take down the sub-objectives, this is much much harder and will require multiple tanks and significant healing compared to the normal situation of going for the end boss after the sub-objectives are met. http://www.wowwiki.com/Av
c) PvE 25 man raid encounters. This will be difficult to pull off. You need to tank, heal and dps at the same time. Many encounters require individual character movement and individual simultaneous reactions by individual characters. Get an overview on http://www.wowwiki.com/Raid. This will be the hardest to pull off.
Lets forget ebay, I plan to level up all my guys myself from level one.

Ok if the undead racial trait doesnt stop fear effectivly then I guess I have no alternative but to use a shaman tremor totem. Can the shaman also take care of healing (if restro), instead of a priest? I see that priest has the group heal and shaman does not but cant I just target each member and cast heals in order or is that way to much mana. Or use that chain heal thing? Well Right now it does seems that Shaman/Priest/3 Mage is the best set up, with one tank in the mix. Still thats 10 defensive characters out of 25.

Lets forget raids, I can see that unless I guild up or do pick up raids that is out.

So that leaves:

Altherac Valley
Cities Attack Halaa seems most interesting
WoLK PvP Zone

Thats a lot to do isnt it? Isnt there someplace where there are lots of enemies gathered (well like 10 heh) most all the time that I can attack, besides cities? Maybe outside 10 man dungons or something?