I would look hard for a factory (westfalia) hitch and a factory trailer harness and trailer module...I have an extra set, and will PM you..

If you buy a used factory hitch make sure you get all the pieces. I see a lot of hitches on Ebay that are missing several important parts that can't be bought separately.

The aftermarket hitches are really pretty light-duty (4-4500 lbs) compared to the factory hitch (6,000 lbs).

A lot of people who attempt to wire in a generic trailer harness to an E53 end up regretting it. The E53's electronics don't like these kind of modifications.

There are a wide number of elec brake controllers that will work. Search on E53 trailer brakes and see if you can find one used.