That seems to be Rubicon's view as well, Ripley / but they've shown increasing ingenuity when comes to providing options. If you think about it, this kit would only need one set of tracks, so the options you mention are about equivalent to what they've done with the M10/M36 (which includes 3 different turrets, 4 guns and 2 hull tops). My concern is that I don't think an SU-100 kit would sell that well, so it's better folded in as an option in an SU-122/SU-85 kit. I may be wrong, however - the SU-100 did see significant service post-war.
Let's see what they do...