over prolonged time running a CPU out of its recommended range causes the die to break down internally. In Windows that means instability/BSODs. It Even degrades further especially when additional voltage has to be applied just to maintain that clock speed. It can get to the point where you can only run it at Stock speeds and nothing more.
You can overclock the non K model but you probably would be limited because of the Motherboards Bus speed. If overclocking isnt the main point of this machine Id go with the Non K Model and save the money. Otherwise if you decide to make it a main point go with it.
Personally Overclocking isnt anything special anymore...