What's the general consensus on the ball joint, replace or rebuild. The manual says replace if the dust cover is damaged, however the dust covers are still available from Nissan. The specs for testing the ball joint are also in the service manual.

I've managed to disassemble, clean and rebuild my original ball joints, any reason for not using them with a new dust cover and fresh grease?

Aside from replacing a part that meets Nissan specs, another factor to consider might be that used Nissan is probably better than new aftermarket.