I wouldn't worry about the heat until it gets in the upper 90's, chickens seem to do ok as long as they have some shade and maybe some damp dirt to lay in (a gallon jug with a couple small holes in the bottom weill keep a patch of dirt damp) For our rabbits, when it gets really hot, we keep gallon jugs of water in the deep freeze then put one in the pen so they can get near it if they want.
If you do put a fan in the coop, I would say an exhaust fan in a window drawing OUT is your best bet. With another window open somewhere, it will create an airflow and draw the hot air out and cool air in. That has worked well in all the barns I know of.