If it's unpleasantly hot for longer than a month, every year, that sounds like an air conditioner is extremely appropriate. The cost is what, half a month's rent to have many year's worth of comfortable Septembers?
in SF, that's one hell of an air conditioner
If you open the windows at night, cool the place down, close the windows and shades in the morning, the house will stay pretty cool throughout the day.
This is hot though :)