If I wanted for an actors outfit to change once the quest reached a certain point, how would I scipt that?

Delphine does this throughout the main quest, but I am not quite sure how to do it seeing how there is only 1 outfit selection in actor inventory.

If you add gear to the npc they will change it next time you load them.
You can also use equipitem to get them to equip it at once

This is what you want: http://www.creationkit.com/SetOutfit_-_Actor