a 96 Pulsar won't be tuned from factory to require a higher octane fuel. Stagea's are (turbo or not).
Platinums for $400? May I suggest US ebay motors for stuff like that? You can get a set of Iridiums shipped for at/under $100.
Pre cat sensor is exhaust temp. Post cat sensor is o2.
From memory, VQ25dd Stagea's use the same cats as VQ35de Stagea's... thus the same cats as the Infiniti G35's in the US.
I can certainly recommend the Berk cats. Made a noticable difference to my car (VQ35de) and if you've seen structure of the stock cats, you'll know why!
They will also be fine for emissions when NZ finally bring in such regulations.
Whilst on the subject of G35's, the intake up to the throttle body is the same as your car as well, so you can use the G35 as reference/parts if you are looking to play around with the intake.