I agree, HK guns are made to last. That includes the firing pin. If it broke, you could always send it back to HK and they would do what needs to be done to get it running again. I don't know how common firing pin breakage is. I've never heard of such a thing before.I don't believe its necessary to replace or buy these parts for spares unless its a duty pistol and you have no backup. My USP is a 03 and I havnt had any problems with mine. If you wish to replace the old style with the new you will need to replace the firing pin, firing pin block, firing pin spring and pin block spring.
Money better spent on ammo and practice.