Use quality CI-4 rated diesel oil in the engine. Specification is more important than brand. ATF was originally specified for your gearbox but it is far better to use MTF 94 and this is now specified by Land Rover. The transfer case and diffs can use 80W/90.
The hole is for a wading plug. Don't put one in unless you're driving through deep water.
Replace the plastic coolant filler caps on the radiator and thermostat housing with brass ones if this hasn't already been done. A new expansion tank would also be a good idea if you don't know the age of the current one.
Change the timing belt every 70000km.
