iissavi: Tell the dealer/seller that a condition of your purchase is that they allow the car to be taken to a reputable import performance workshop for a thorough checkover prior to laying down any funds.
I sold my GTS-T a few years ago and Jim at CRD in Silverwater inspected my car for the new buyer and I was incredibly impressed with the level of detail they went to.
They checked the engine, body, every switch inside the car, radio, road test, braking, suspension etc etc and dynoed the car to ensure everything was in good working order.
Expect to pay around $200 for a full inspection, and believe me, its money well spent.
I normally speak to John in parts at Nissan
You may be able to try the parts division at Crows Nest Nissan as well