Hi, and welcome to SAU, the best place in the world for all things Skyline.
According to the Victorian "P" Plate laws, any vehicle (Even Mum's Camry) can be considered prohibited if it: "has an engine that has been modified to increase the vehicle's performance (other than a modification made by the manufacturer in the course of the manufacture of the vehicle"
This means that exhaust mods are basically off the table for P Platers, as even a cat back, like you mentioned could theoretically give performance gains. Still, many do it anyway. (A cat-back on an N/A would give very small, even possibly non-existent gains)
If you still want to do this, try to be a bit stealthy about it. (An R34 with P Plates on it is already a cop magnet, regardless of modification level)
I personally suggest ringing up or emailing your local exhaust shop, and getting a quote for something made up and installed, as most popular brand made bolt on systems systems are for the GTT (Turbocharged) model.
Completely avoid the cannon, and try to stick to the stock twin tip look, to avoid attention from anyone unwanted.
Maybe look into getting an X-Force Varex muffler, which allows the volume to be controlled if you want to keep quiet if need be, to avoid unwanted attention.
The RB25 sound can actually be quite nice, if done properly.
Good luck with your R34.