The Wesfalia hitch is documented on the Westfalia site in Germany. Good news for those that don't read German, first select the Union Jack flag to get an English version of the site. Then, select the Product Finder and Installation on the left side of the page.
Choose BMW, and X5.
Note that you will have options for E53, E70, and E70 with the MSport kit. There are also kits that have the wiring included, but for a North American installation you would order the NA wiring kit (without hitch) from ECS Tuning, a board sponsor. The trailer wiring plugs are different standards for Europe.
Select which hitch you are interested in, and select 'show details'. This has installation instructions, etc. There you go, more info.
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There is a note on the Westfalia site that the E53 in North American spec does not fit the Westfalia hitch kit. That is likely due to the impact-absorbing bumpers, as the BMW Westfalia kit includes an alternate crossmember to clear the hitch structure for the NA hitch design, and there is likely a similar interference problem here. Or, it may be that it fits but is not compliant with US crash impact regulations.
Both hitch models are rated 3500 kg (which is 7700 lbs, similar to the 7800 lbs limit BMW listed for the Euro spec E53 years ago) Note that a heavier duty hitch does not override the manufacturer's rating for the vehicle, so you would still have a 6000 lb limit given BMW's published guidelines.
Your next issue is to find a Euro supplier that will sell it and ship it to you.
Then, you need to determine how to attach safety chains. In theory you could weld loops on the swan neck, but given the swan neck is attached at one point, it would be better to bring loops down from the tow bar cross piece.
You will be limited to a 2" ball, given the 50 mm size of the Euro hitch. Should not be too much of a problem.
I would stay away from the electrically activated fold-out model, unless you are prepared to wire it up separately. Vehicle coding in North America may not support that option, I don't know.
If you ever want to use a bike carrier, you either have the option of fabricating an adaptor from the Westfalia receiver to a 2" square receiver, or purchasing the rather expensive Westfalia bike carrier.
Good luck, and let us know if you try it out. Many have considered it, and given the cost, decided they can live with a 2" receiver.