Welcome (soon) to our humble little town.
Technically all you need is an ID inspection, BUT if they suspect the car is modified they can send it for a full inspection. I dont think re-shelling is required (as mentioned), BUT you will want to have the car stock looking. If you have some photo's to see what it looks like will definately help.
Drags is dead in the water (as mentioned) but circuit and drift is still pretty active. We only have the one track Mallala, which is a pity, but its usually pretty busy there with practice days, club sprints etc. check out www.mallala.com
Workshops recommended in adelaide include Boostworx (for all things nissan) Morpowa, Southern Hi-Tech Dyno, etc. Plenty of choice with parts being just as avaialbe as the rest of aust.
Definitely idiots and enthusiasts. For enthusiast best joining actual clubs that participate in events, eg. Club PSi, Nissan Datsun Owners Club etc, forums tend to contain more idiots than enthusiasts.
If you can afford it, eastern Suburbs like Norwood, kensington, magill etc. Or inner south such as Unley, Clapham, even Blackwood etc are all good.
Again, welcome B)