A curiosity: how much time (on average) is required for each car preprocessing? How much for legality check / rendering / CFD preparation?
Generally speaking: I am convinced that sketchup gives some advantages in the short period (it is easier to learn), but the use of a real cad (producing not faceted geometry) would be better in the long term for both partecipants and organizers (FreeCAD, for those who like parametric modeling, or Blender, for those who prefer direct modeling, for example, are free and open source).
To prepare and repair the geometry for CFD starting from a STEP would be easier, I think, for Chris (and the stl would more uniform if generated by a professional CAD, by the organization).