Welcome G35 Toronto!
As a buyer I always look to make sure there are absolutely no modifications. I mean not even a Z-tube!
I haven't had any problems with my 2004 G35, only thing I had replaced were some gas flapper valve that I couldnt fill up my gas tank to full without the pump stopping and also my rear view mirror would shake. All replaced under warranty.
Are you looking for a manual or automatic?
Its pretty much common sense, I would normally buy from a lady since most guys would probably push on the car more then a lady. Ask to see if they have service records, car fax report, any work has been done on the car.
You can look at service bulletins as well
http://www.infinitihelp.com/In...r.htm
and also look at other forums, not trying to discourage you from NICO because NICO rocks, and just see what other problems people have had.
I do recall some 2003 G35's having some transmission problems and brake problems for the non-brembro models.
Hope that helps!