The 350z is pretty bulletproof apart from a few well documented issues. If you know what to look for you will be fine. Dealers, including Nissan dealers often havent a clue what spec the car is even when its standard.
My advice, as has been said, get to a meet and you will be able to have a good look at the car and talk to people who understand it.
Also, if you find a car which is a fair distance away, get a local to check it out first to save you a long journey. There are some great cars for sale on the forum and you can search the members posts to get a feel for the car. There are also some real dogs out there.