If I was in your position, i would make an integer/real variable for all of your heroes.
Lets say SpellHer
ne is for your first hero, and lets say that he gets an item that has +10 spell damage.
Make a trigger that says Set SpellHer
ne = (Spellher
ne + 10).
Then I would make triggers for each one of my hero's spells, that basically says:
example: Holy light heals 400 damage (If you have levels of spells, set them per level)
Your trigger will be
A unit starts the effect of an ability
Ability being cast equal to Holy Light
Unit - Set life of (Target unit of ability being cast) to (life of Target unit of ability being cast + 400 + SpellHer
ne).
Then you can add a floating text line here over the target unit of ability being cast to show that they got however much bonus healing done to them.
It might sound like a lot of work, but once you get every hero and spell level worked out its just a matter of plugging in numbers.