Triggers or AI script?

Hello guys :)
Before i start making the map, i need know how am i going to achieve this.

Concept:
There will be an empty arena. When the times is up, the enemy base and enemy units will spawn by triggers. (Note: Arena will spawn in waves, different race and units)

I want to make the AI to rebuild the dead units. (Buildings are given)
Eg.
Wave1: If it starts off 10 grunts, i want to AI to have no more or try to have no less than 10.
Wave2: 10 headhunters (new AI, i dont want them to create grunts this time :|)

How would i make the AI flexible like this?
Triggers looks like an ideal solution to me, but i dont know how.
Or do i have to create a whole AI script for this?

Thanks in advance!

Well since i don't know anything about AI, I'd just make it with triggers, it doesn't sound that hard.