compliance is a VERY technical process that can take as long as a year to be authorised by DOTARS. keep in mind you have to prove that the car you're bringing in as a grey import meets EVERY single ADR (including emissions data for the new VR engine, crumple zones, and protection etc). Once you can prove this, then you're given permission to import them in volume, which is how you get your R34. Usually one workshop collects all the necessary data, and once they are accepted by DOTARS, they usually onsell this data to other workshops who want to do the same. The other workshops don't have to crash test a car, and thereby save money, the original workshop that accumulated the data recuperates the costs by selling a stack of photocopied paper for thousands of dollars... win win situation.
but in any case, compliancing is easy in the latter stages once somebody's already done it, but if you're pioneering it, its like drawing blood from a stone and can be a very long, stressful process.
That said, putting in the groundwork now in the early stages (ie before Nissan sells locally) means that few years later down the track it'll be worthwhile.... just like compliancing S15's is now.