From my knowledge, this is what i have to have:
Check the spellcaster flag in datEdit
In iscript, give an animation for spellcasting
in Firegraft, actually give the spell to that unit, and change the spell's dat.requirement...

anythin else am I missing?
How do I know if the new spellcaster unit has a energy point of 200 or 250?

It should be 200 as default. And the little bar should actually tell you when it's filled and how much energy does it have.

Did you actually try this before posting? It seems correct to me.

Yep, that's everything. FYI, a unit can still cast spells without "spellcaster" checked. All it does is display the energy bar and makes the unit a valid target for things like Feedback.