[kaisen]

In Europe, they don't have pickups. They tow with cars even smaller than a W124. So in Europe, it was a Mercedes-Benz option for the Oris (o.e. brand name like Mahle or Bosch) receiver hitch with removeable swan-neck ball-hitch. They use 50mm ISO balls there which will work fine with 2" tongues here (a touch small, but works fine), or they can be machined down to 1 7/8". The Oris hitches were rated for 4200 pounds on a W124. Your wagon will handle a decent load if your smart with your tow style. Remember that anything over 3500 pounds is legally required to have brakes, and I'd personally recommend electric brakes (not surge) on anything over 1500 pounds.

I'd search ebay.de or some Euro sites. It ain't gonna be easy, but it will be pretty cool.