attack target

Is it possible to increase a heroes mana regen while when they're attacking or being attacked and for it to last a few seconds after they stop? If so, how? If possible I want to do it with just triggers. If not that's okay too, it's just preference as I'm fairly new and don't have any...

I am not sure I'm posting this in the right forum, but here it goes: How do neutral hostile units (aka creeps), or, for that matter, computer-controlled player units choose their attack target if surrounded by more than one enemy? Do they check armor, level, type (building, unit, Hero, ward...