Hello,
A squeaking timing belt would be very bad on any car... But no the Z doesn't have a belt - it's chain driven.
As you say, the most likely source of the start-up squeek is one of the aux belts (probably the alternator belt, but always worth changing them both). The two belts are the same on all models JDM and UK, part numbers are 117204P102 (alternator belt) and 11920AG900 (a/c belt). Should cost around £35. Give you local nissan part store a call.
Also check out my how-to guide for swapping them over, in the guides section: click here.
Steve
thanks for the two part numbers! your linky to guide isnt working btw.
A.