TOP down is a good approach to use in designing any type of system (like a gearbox). However, in SW you really have to understand the implications of the actions that you take in using the top down approach. In ACAD you might make your layout and then copy a bit here and a bit there as a basis for a detail. In SW there is no good way to make your layout (which you later made the assembly) prior to finishing the details. This can be an aggravation and can slow down the design process. We struggle with this every day because we do quote drawings and then have to start from scratch again in SW to actually make the model. Nevertheless, it is possible to approximate your layout methods in SW, though you won't be able to use them as the final assembly drawing by adding a title block.
The implications of doing anything top down in SW is that you create in context relations between the assembly and parts or between parts through the assembly. This means that the components involved in any in context relationship must be solved each and every time SW does a rebuild and that the relationships between the files must be maintained so that there are not circular references, etc. As in life sometimes a strength can become a weakness. The strength is that if the assembly changes so does the part. The weakness is the overhead which requires a lot more thought and resources. The practical outcome is that most people use in context to enable copying features from one part into another and then they remove the in context relations.
There are other methods that don't require in context relations and these involve envelope parts and skeletons. These are very simple parts that carry key features of the layout and can be used both in the assembly and in parts to transfer layout information. These methods are faster and more robust.
Since you are well versed in ACAD let me propose a method that you might find familiar and yet transferable to SW. Suppose that in DWGeditor or in ACAD you created your layout as usual. As you need to create details from the layout you bring those portions of the ACAD drawing into SW via 2D to 3D having previously placed the detail features on a layer to facilitate the importation. You can then use that geometry directly as the basis for a part, or you can create surfaces from it to use as a skeleton in both a part and as an envelope in the assembly. Much more elaboration on just how to do this is needed, but I won't take the time here.
Once you have all your parts, make the assembly, check fit and interference and create your final drawing from the assembly. The most obvious reason to do this is that SW will allow you to create section views and auxilliary views as well as your axonometric projections much more quickly.
One thing you will probably find in using SW over ACAD is that in SW you will start finding things that you did in ACAD that just don't make sense in 3D. That is one reason it is worth the effort. You will also find that it takes you a bit more time on the design end to do it
3D, but that if you factor in the time saving from having parts that actually fit the first time you will be ahead.
It is a shame that SW never considered making drawings able to parametrically drive components like some other CAD packages but it is more an aggravation than a roadblock.
TOP
PS before doing top down, read all the help on incontext, envelopes, importing parts into parts and 2D to 3D conversion.