I actually considered all of the major East German politicians, but I didn't want to pick them for a few reasons, the main one being that I wanted a character who is a bit more sideline. Picking one of the major leaders not only makes a bit of a statement, it also forces me into more of a rut with them, since they were major leaders with more well-defined paths and traits.
Selbmann was appealing to me because he carries a very Göring-esque biography, where he moved between different positions of relative power and always seemed on the cusp of greatness, but was always overlooked due to circumstance. Simultaneously, he is adjacent enough that I can bend him as I need.
As for the election, that is more due to the relative difficulty that the SPD has in choosing a good candidate. Sudden elections and candidate expulsions tends to cause a lot of strife, and of the factions within the SPD, the KPD is the strongest. There are still a lot within the SPD who aren't fans of Selbmann, but they see his leadership as a skew, not a complete alteration of the party. The SPD has, till this point, rested heavily on the likes of Wels, who basically chaired it for most of the Republic. Replacing such entrenched leaders often causes upsets, and I think the party choosing him is plausible if they feel that he is willing to fight against the decay of their democracy.