you wanna watch fan thickness too, likely that a tornado wont fit inside there. but any standard thickness 80mm 12V fan will do the trick. you might (will) have to cut/splice the wires, but that isnt a problem long as you tape them up afterward.
edit: and on safety, obviously unplug the psu before opening it, and I've always used a screwdriver (with insulated handle) to short the 2 big heatsinks in there to the metal case. only place i've ever been shocked inside a psu was on the heatsink closest to where the mains enters the board, usually the left one if you're looking at it from the open top with the fan on the side closest to you. It is possible to be shocked elsewhere, just unlikely and it's never happened to me. and since you're just cutting 2 wires and splicing to them. there's no need to remove the circuit board unless you choose to unsolder / resolder the wires to the board, assuming they are soldered to it as on most cheaper PSUs.
