I take you have a Cossie YB engine.
You'll need a T5 gearbox to take the abuse as it were. I think the clutch fork bearing is different but clutch flywheel and pressure plate can be left.

Yeah 2wd sump and oil pick to change.

Loom wise stick with a 4x4 loom. Have you got the right ecu (L8)?? Plus a Bosch 044 fuel pump minimum.
A Cossie rear diff depending on what power you will run.