These are the import formats supported by SketchUp:
A quick google shows that converting STEP files to something SU can use is doable but not perfect. There is an SU plugin but it costs a lot of money for casual hobbyists.
I would mainly like to see chassis and related components in 3D, such as motors, wheels, casters, hardware and the like. That way we can figure out exactly what we need and what works together before ordering. Of course the more generic items might already be in the SketchUp 3D warehouse but it would great if items have a link to the warehouse in their resources tab so that we know for sure that the model has the same spec as the part.
Anyway, it's great that you guys are interested in supporting 3D modeling for items. I think it adds real value, even if at the moment there are not a lot of people using modeling to build their bots.