Hey goedhart,
It's all good if you go to a good U-Haul store with an installer that is decent. I just went to the biggest one in the area with a nice shop and they ordered everything and installed it the next day. (I was in a little bit of a rush)
They sold me the Hitch, Receiver, Tow Ball, & wire harness including and extra extension so I could tuck it back in my trunk when I'm not using it. I don't remember the exact cost but it was less than $400 with everything installed.
Good luck and lemme know if you really want some pictures of my install. It should be all good though if you go to a good Uhaul store.
P.S. You shouldn't need a extra harness or module because the kit they have is self-powered and just taps off of the existing lighting wires. The BMW kit is really easy and requires no splicing......just like the sissy owners need.....be a man and splice those wires dude......or pay a pro to do it like I did

LoL!