Interesting "Paper Panzer" Ripley. They only made 5 of those as a first prototype "proof of concept" for the Stug III. They started out with wooden mock-up guns, later replaced with the real thing. But the armor was soft steel. They were considered unfit for combat, and only ever used for training purposes.
Hulls were built in late 1936, where they were then sent to Krupp for mounting of actual guns in early 1937. By the fall of 1937 they were used in trials. The original version of the prototype had no roof, they were open topped. During trials, they decided this was not going to work for their intended role, and a thin roof was added in 1939.
Ultimately, the prototypes were successful, and that led to the first production variant, the Stug III A (based on the 6-wheeled Pz III), produced between January and May 1940. A total of 36 of this variant were made, before they moved on to the Stug III B.
I would definately pick up a Pz III D if one ever becomes available. They served in Poland and Norway. Don't think I would try and Model the prototype Stug though. That looks like a LOT of work!