Now the questions is:
1. Is it worth the risk buying a working one, only to maybe have it fail on you?
For your F-mount cameras, I feel it's a worthwhile gamble. In fact, a couple of years ago I paid a considerable amount to have the motor in mine replaced and the lens refurbished. For the Z6, I'm not as certain. While I've used mine on a Z8, more modern lenses like the 14-24mm and 17-28mm outperform it.
2. Does this problem affect only certain [earlier] batches (the problem was fixed at some point)?
As far as I can tell, it's a problem endemic to the model. Mine started squeaking twice. The first time it took about 18 months from the first squeak to failure. The second time it took about a decade.