I think you might be better off selling these upgrades as a complete kit, rather than as separate sprues. That way, people can build a vehicle straight from the box (I have a feeling that people might be less likely to buy a set that doesn't include the actual vehicle). Then, when they see all the other cool stuff on the sprues, they'll buy more basic vehicles to build the other variants. But I don't know if that works from a business perspective.
If you took this approach, it could work something like this:
- a 3-in-1 set for the Ausf C, which includes the basic vehicle. This would include the 'Ausf C only' versions (although you'd make it clear on the box that these parts are also suitable for the Ausf D). Perhaps it could include the /3, the /7 and the /9 "Stummel", since that would probably fill a sprue (especially as it would include an optional upper hull). I think including the /3 and the "Stummel" will make this a very attractive kit - having either version in the box art would (I think) help sell the kit.
- a second 3-in-1 set for the Ausf D, which might include the /2, the /9 and the /22. Presumably if this includes the early version of the /9 (again, with an optional upper hull), it would easily fill a sprue. Maybe market this with the /22 version in the box art, since it's visually striking.
- sell the /16 as a separate kit, as you have with the Stuka zu Fuss. Maybe market it as an Ausf C, since Warlord have already put out the Ausf D version (but mention on the box that it's suitable for the Ausf D as well). As you've mentioned, it could be a popular vehicle with gamers, and yours is going to be better than Warlord's.
Just some suggestions.