Theres no real 'tips' for the cars... Just test it like u would any other car and dont rush into buying one! main tip, dont buy a $15000 car if you have $15000... Make sure u have cash for servicing, repairs, insurance, rego etc....
If you search u will find lots of information, useful too... Main differences apart from looks that i know of are Airbag and ECU but u will find lots more in this topic...
http://www.skylinesaustralia.com/forums/in...=33+differences
As far as importers... Depends where u are located... In melbourne i would reccommend Kamikaze motors. but in the search option type 'importers' u will be sure to get a few topics.
The fact of giving someone your money and sayin buy me a nice skyline doesnt work that way...
U tell him what price range, year, colour, condtion, miles, accessories everything and he will find it... a lot of places will take a deposit, get what u want and then when it comes in you can say no if you are not happy with it.
Try looking in the importing section on the forum. I actually havnt looked there but sure to help u out.
Good luck with everything mate and have fun shopping