If you are getting a 2500k, there is no reason NOT to overclock. It's a free performance boost! Many mobo's have an "easy overclock" In which it at least bumps the speed a little bit. I suggest you at least try it.
And yes, a 120MM cooler (Like the Hyper 212+ or Scythe Mudgen 2100) Would have no problems fitting in the HAF 922, none at all.