First off, Nissan never offered the Z33 with a Torsen diff, so it's an aftermarket unit. The OEM 350Z came with either an open diff, like yours, or a VLSD, which uses different stub axles. So your seller will have to get his story straight, or you need to understand what you're buying.
As for whether it's worth it or not, only you can judge. I sure as hell wouldn't be buying a LSD if donuts are what you're after!