There are usually one of two reasons to rebuild the shift-shaft assembly; the machine has gone down heavily, or it is has simply worn out spoiling the riding experience with missed changes and a sloppy action.
Whatever the reason for a refurb, the same principles apply, in so much as the more parts you can afford to renew, the better the results. Less play, less sloppiness and a general feeling akin to a rifle bolt sliding seamlessly in to the breach.
This component and those listed below may well be on your shopping list?