Start Here:
http://www.edmunds.com/tmv/used/nissan/index.html
Once you find the car you want see what the true market vaue is and then see what kind of condition etc. the car is then post us back and let us know what your dealing with and maybe one of us in the forum can be a bit more specific. Good luck we look forward to slapping a 350z badge on your chest.
I paid $37k for my 350 GT. It was marked at $41K, but they dropped it and put tint, clear nose bra, NISMO Cat Back Exhaust, and Dual Air Intakes, I guess they wanted to get rid of it. They told me everyone that looked at it wanted automatic, I was the only person that wanted a 6 spd. So they thru in all the goodies and it has every option available. It is basically a black NISMO w/o the #badge. I love it and people compliment it all the time.