To start you off - there is a thread by Merli - its a sticky and has all the stuff u need to look for when you go to buy a skyline -> here . Next, go to SSV in Oakleigh and have a look at the cars they have there. Cars there are excellent. Speak to Mark, he will help you out. And finally, i take my car to RE Customs (springvale) for its service and mods - Ray is a member of sau (SXR), and he will make sure your car is running without any problems.
Got my R33 from Wayne at GTAuto Compliance in croydon - he does the compliance work for the cars that come through the J-spec guys i think. You can talk to him as well, he will give you good info. He spent heaps of time going through the ins and outs of buying an import. Def worth speaking to him as well.
My advice is to look at both dealers/importers and private sales as well. Private sales, you can get some mad cars with heaps of work done for a good price. There was a white r33 with heaps of mods, and went for 23k or something (series 2). Im pretty sure the guy who owned it is still a member here - he has a celsior now i think.
Dealers/Importers give u the 3 year warranty - not needed if the car is in great condition - but useful to have in any case.