I had a look, but if I properly understand the way I shall move from STEP assembly to the new Part structure. A part might be able to contain multiple Body. And part can contain multiple parts. I got an issue currently which is when I want to move a body from a part to another it does crash. Is it supposed to work ?
It should not crash
The moving should actually work, but again, still working on it. It is rather rough at the moment. However, as you are working on this now it may be good to use the Parts to not need to make the switch later.
By the way: Bodies are only for PartDesign features. For step import i think you create normal Part::Features. Those can be added directly to Parts (as well as everything else, no restriction)