Some thoughts on this. As I understand, money and time are irrelevant.
I will therefore only share my thoughts on class setup, control and purpose.

Purpose
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.

Group Setup
Ranged damage dealers are the key component in your group, but you should forsee healing and tanking as well. For group healing that can be done with one button and requires no targetting, take five priests, one for each group. I would also add at least one paladin tank, these are best suited for multiple mobs and do not require more actions per minute as the number of mobs increases. Note, tanking is PvE only, you can not tank players. To decide the setup of your 25 character raid group you should also consider the different types of buffs. There are self-buffs, buffs for a friendly target, group buffs and debuffs. For example the priests can use http://www.wowwiki.com/Power_Word:_Fortitude to buff the raid's stamina/hit points, while the mages do the same for the mana points. Then there are party wide buffs, like paladin auras, shaman totems, Druid Moonkin 5% spell crit aura and a number of others that affect only the five players in a party. There is talk of this being changed to a raid wide effect in WotLK. Finally, there are debuffs on the hostile target. Shadow priests increase the damage taken by the 5%, Warlock curses reduces resistance against magic damage types, increasing damage. Your paladin tank also can offer 3% more crit for each spell against the target, mana returns and some others. You already said that you want to wait for the expansion to see what happens with the group buffs. Considering however that the buffing classes do not just buff, but also provide comparable damage output themselves, that is not so critical. The power leveling process works only for up to 5 players, so if you need to pull up a few characters later, that won't be of any real impact. Today, I would go for a setup like this: 1x Paladin-tank 5x holy/discipline priest, 5xMoonkin, 5x elemental shaman, 1x warlock, 8x mage. If the group->raid buff for WotLK is true, you could go for 1x Paladin-tank 5x holy/discipline priest, 5xMoonkin, 1x elemental shaman, 1x warlock, 12x mage. Also, the new death knight does some frost things which might be in good synergy with frost specced mages, but that is TBD. Be aware that for 25 man raid dungeons you need at least two, sometimes three or four tanks per encounter.

Control
For WoW, there are no macro's with delays, and using delays in game input devices, programmable keyboards and whatnot is not permitted. It will get you banned. The default damage caster needs to cast a new spell every 2.5 seconds. Toggling through the characters is therefore not effective. You want have simultaneous control input on all characters. There are different hardware and software ways to do this. As I see it, the best way in your case would be five quadcore machines to run five clients each and handle the distribution of input to all windows via software multiplexing, for example via KeyClone. This software is the best known one at Blizzard's side and nodded off. No seals of approval or anything, but your best bet. Then have a vetra hardware multicaster to broadcast input to all five quadcores. If you want to go all hardware, cascade some vetras and use micro-PCs. Pentium-M notebooks with discreete graphics do work as minimum spec example. However, control and maintainability will be best with five quads. You will also need considerable amount of keys. A normal keyboard, plus some X-Keys pro and maybe even foot pedals would be my expectation. Using Multiplicity from Stardock you can move your mouse and main keyboard over all computers and clients - if you go for five computers, their solution does not allow more than eight PCs.

My 3 cents. =]

Kind regards, Steph.