you can always get hole saws , like this for a clean precise hole . its either a 2 1/4 or 2 1/2 cup for an 80mm fan but can always be measured to know exactly .

I would mark out the position of the fan , then drill it , then take one of those premade fan guards and place it over the hole and mark my screw locations to mount the fan .

You only use 2 ho9le to mount fan and opposite 2 holes to mount the guard unless you have long enuff screws for both

good luck with the case modding , and yes please be carefull , my first mod cost me 275 bux in the emergency room , metal shaving in my eye , so use saftey glasses