Yes you must buy that fan, make sure it is only 80mm and not 120mm ! You also need an intake fan at the front if there is room for one, plus i advise you to clean all that dust out and put a dust filter on the front intake fan.
Ok, I'll buy the suck-out fan. Sadly there is no room on the front for an intake fan, but there is a 80mm duct on the cover that I can remove. I can remove it and place a intake fan on it, would that work?