Your thoughts regarding the lights and charging are pretty much right. But assuming it weighs over 750kg then yes you will need to get a brake controller for the electric brakes. Your D3 will not have one unless you or a previous owner fitted one.
Generally the details on the forum are for permanently fitted brake controllers and this may not be your preference given that you are only hiring the camper. But I don't know if there is a more transportable or interchangeable option. Perhaps check with the hire company.
If you go for a permanent fit there's plenty of details on here (as you've seen already). Level of difficulty is a subjective thing but if you're not confident, perhaps look into getting an auto electrician familiar with the Disco to install. You'll be up for probably $700-1000 at a guess.
Regards,
Scott
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