"It seems to me that your problem is not a fan, but a poorly thought out power supply."

Yes, that was my immediate reaction, too!

Have you thought of putting the PSU into a separate unit?

"The problems with a fanned unit is dust"

From that point of view, back to your original question, it is best to have the fan blow into the unit, and fit a dust filter on the inlet side of the fan.
But then, of course, you have to remember to clean the filter periodically...