ok, well, let's see...
as for your first part of the question
i'd say that you can just use the warpgate (either take it's ability, or maybe even make a copy of it, and just change the model (and any other stats as necessary))
hmm...
to select a unit to become a hero?
you mean to select a hero?
or you mean to "upgrade" a unit into a hero?
if it's for selecting a hero, then you can either utilize the event "Unit - Unit in range"
or you can create a region that overlaps the same area as the circle of power and use the event "Unit - Unit enters region"
on the other hand, if you meant something else other than the ways that i've answered, then please rephrase it, and ask again
hope that helps