Nighttime Ventilation questions for newbie..AC or Fan?

You could do a little exploratory surgery on your vent without an exhaust fan. Simply remove the screws holding the shroud covering the hole in the ceiling, it comes off pretty easily. The shroud is easy to put back up. Then look around to see if there are already some wires running through the area. If there are wires there you simply tap into them to power the new vent fan. If there aren't any wires, I'd upgrade the fan in the bathroom. You would be surprised how much air on of the upgraded exhaust moves.