If your going to change the timing belt, I recommend changing the tensioner and the idler pullys (and new tensioner spring), both cam seals and the crank oil seals, water pump.
If you are going to change it with the engine in, it will make getting the harmonic balancer easier to get off (they can be done up to 500nm), I recommend removing the radiator if your going to remove the harmonic balancer bolt with a extension bar against the chassis and quick cranks of the starter motor (700+ cca battery is handy) to free it.
I would probably leave this up to a qualified mechanic (even then I would only trust one that has worked on skylines before as I've seen qualified mechanics f**k up a timing belt change), the dangers of doing it wrong is all your valves smashing into the head.