Mana buff

how do i go about creating a spell that for set 3 seconds that unit has a mana buffer of

500 adding on to unit's current mana.

Mana cost expend will be reduce from that mana buffer.

After, 3 seconds duration that buffer will be removed from unit's current mana.

needed some guidelines on where to start

Create a trigger that detects the spell cast. ( lets say berserk, I like it because it doesn't interrupt unit's orders.)
Increase the unit's maximum mana by 500 (be sure to restore mana equal to the mana max added as it doesn't do this automatically)
Add the unit into a unit group
Create a second trigger that fires every X.XX seconds
Check if the unit in the group still has the buff, if so do nothing, if not, remove 500 max mana!

This allows the unit to take dispels if you choose a "dispellable" buff.
If this is too difficult i don't mind making it.