Making a mage (noobs first unit, please help)

I want to try to mod a few units so I could create some neat custom battles with armies from a Swedish Viking inspired fantasy RPG.
Since my whole experience with modding is that I fixed the horse archer bugs in 1.2 you can say that I have no real idea on what I'm doing, but I'm willing to learn and I think I understand the basics. So some quick questions (and I know I should probably use the Caligula program):

a) Which files should I backup before I do something very stupid.
b) I want to make a mage and after reading the forum I've come to the following conclusions on the steps I need to take.
1. Find celtic druids or similar unit. Change unit size to say, 1 person.
2. Change attack values to the onagers fireball thingy (including I assume range and power of attack?)
3. Change hit animation to the onagers hit with fireball thingy, if possible simply don't have animation for flaming ball of mass destruction actually traveling over battlefield.
4. Change attack animation to druids chanting.
5. Retain ability to chant for bonus morale.
c) Which file(s) do I need to change, and does caligula find them for me? (If so massive love to however made the program.)

So after doing these changes should I have a unit that looks like a druid and throws explosions around the battlefield, and which can top it off it can boost morale on surrounding units.

Additional questions:
Is the sparkly star thing that happens at time avalible, could you possibly add it to an attack animation?
And of course am I forgetting something crucial?

I know this probably sounds stupid but I usually learn by doing something rather odd and difficult the first time, just adding hopolites to the Juli campaing seems stupid, because this way I can ask for advice once and if I pull it through I should be able to mod almost any kind of unit that I want after that. :P

for example as i see in lotr mod, there are a unit hidden in a omgtwf faction that are fire templars, they lauch a "mage" before attack... are a flaming arrow, you should ask them

I made fire trowing Aes Sedai a long long time ago.

Basically I used the javelin animation, gave them the projectile for flaming catapaults and there you go!

Fire throwing mage unit!