You don't say why you think yours needs rebuilding, so I'll just make some general comments.
I wouldn't say there is any reason to "routinely" rebuild the Datsun gearbox unless it's acting up or sounds bad. The things to inspect are mainly the ball bearings, synchro tolerances and end play (thrust washers) It's known that there are some inferior synchros offered, but I don't know the source. If you end up with a set of those, you'll be worse off than your old ones. The only bad ones I have encountered are ones that were rebuilt by someone else, including "professionally"
One issue is with ball bearings You want to check the bearing cage as they sometimes disintegrate and only the balls and races will remain. I have seen 3 or 4 that way. Ball bearings are available from the Nissan dealer, but they are expensive. The last set of 4 I purchased was about $250. Most parts are not available through Nissan (synchros, of all things, are not) and depending on who you talk to, some counter guys will not even bother to look up parts for something that old.