Drive it, enjoy it, worry about the trans when something happens - K's really aren't everything, service history plays a big role.. If it does go pop, just throw in another, or even try find a 5sp STi box with the right ratio to match your rear diff and get back to it.
My car has done 147,000k and 4th gear is crunchy, not the end of the world and it still gets a beating when I drive it.