The first thing to do is figure out what model you are wanting. MB make NO wagons before the 123 but there were private firms that converted them. Mostly for delivery service or ambulances or funeral cars.

And quality varied from one converter to another.

I do know the US customs will not allow, under any circumstances, the import of any ambulance built on a passenger car.

If you get one get the best you can find even if the cost seems a bit high. It will be the cheapest in the long run as the special wagon parts for a 111 are through the roof.