I have done a 200TDi alternator, actually it was off a ford tractor that happened to have the same Lucas alternator as land rover use on the 200's.
If they fit the same Lucas unit to the 300TDi then the diode pack and brushes come as one sealed unit.
It’s just a matter of disconnecting the cable, removing two bolts on the diode pack and the unit should pull out.
Then the reverse for fitting the new unit. Can be done on the vehicle.