Bob,
The more I look at your original photo from Caffiers, the more convinced I become that it's a 320 Kübelwagen. Very similar vehicles were built by other firms, for example Adler (the 3gd) and Wanderer (the W11.)
Here's a 4-view drawing of a Mercedes Benz model 320, which I presume was modified into the Kübelwagen.
Someone called Merlin on the website Britmodeller.com is attempting to build a model MB 320. It might help you to seek out and read his thread in the website. He considers using the wheels from a 1:35 scale Steyr lorry in his build.
I look forward to seeing pictures of the finished model.
Rob.