You certainly need a wiring harness, however most vehicles are pre-wired. Which means that there's probably a plug under the dash to plug the brake controler into and a plug somewhere near the rear of the vehicle to plug the couple of feet of wire that runs from the hitch mounted 7-pin plug. It's not like you actually have to have new wires run from the dash to the hitch.
You may be able to go with an aftermarket kit, like this:
Range Rover Sport Trailer Wiring Kit (Part # YWJ500170)
Or you may be better off getting the OEM kit. But you can get the kit and put it on your self or have somebody other than RR do it.
Same with the hitch, you can get OEM or aftermarket and put it on yourself or have somebody else do it.
"tow mode" may not be BS as somebody else posted. From what I know about VW's: Today's computer controlled vehicles are smart. When enabled, "tow mode" changes the shift points of the transmission for example when the system detects a trailer is attached. When you get a vehicle from the factory with the tow package, they'd would have enabled this. Since you're adding the towing package after the fact you'll have to get it turned on. You may have to get this done at RR (and you might need OEM wiring harness for it to work). Although, even there all you need is the right tools (software/hardware connected to diagnostic port) & knowledge and you can turn it on yourself.
Factory tow packages usually include a transmission cooler. But for instance, all Honda Ridgelines include a transmission cooler even if they don't come from the factory with hitch and harness. So you may or may not need a cooler. Again, OEM or aftermarket is a decision to make.
I suggest poking around here:
And see what other RR owners are saying. If it's anything like the Honda Ridgeline forums, they will be able to help you understand exactly what you need and be able to guide you in the OEM vs aftermarket choices.