Please correct me if i'm wrong guys!
There are 3 models of the ZED that i'm led to believe, 03 - 55 plate are the originals, 06 - 08 are the second models as the 55 and below had a few problems with a main one being the clutch... and the 08 onwards, not too sure on these..
IMO spend a bit more and get a newer model. the ZED's hold their value quite well!
There are three model years (MY) of the 350Z.
MY03 (or 02 if import) being the 280ps first release.
MY06 which has 300ps and a redesigned front bumper and could of bits on the interior
MY07 which has the 'speed hump' bonnet, 318ps (IIRC)
Known issues can be seen in the FAQ and warranty issues. No clutch issues on any of the MYs as far as I know, and no one MY that has anything noticeable between them really. The latest was the only one that was meant to actually be any quicker as the MY06 was actually heavier apparently.
As for what to buy, get to see them and drive them and go with a list of checks and you'll be fine.