Use the Nissan USA website
http://www.nissanusa.com/ to learn what the differences are between trim's.
Mostly it is
Base = no LSD or HID's comes with 17: wheelsEnthusiast = no VDC or tire pressure monitoring and comes with 17"wheelsPerformance = no Bose and comes with 18" wheelsTouring = leather, Bose and 18" wheels on the MTTrack = 18" Rays, Brembo's
05 the Track and Ann. edition(MT) gets *new 18" wheels and Brembo's as well as a bump in HP(but less TQ)
There are a few more differences I'm sure that I forgot to mention, so check the website.
When looking at used 350z's you need to know more about the car and options than the salesmen (which in most cases won't be hard).
Post up any more Questions that you may have and I'll get you the answers.
BTW: each year Nissan has offered a limited color on the 350z.03 - Brickyard (IMO the best color)04 - Liquid Aluminum05 - Ultra Yellow