The hard thing with DXF is that you'll have to rewrite the whole file.

Creating a Bounding Box of everything in the file can easliy be done with a BoundingBoxAccumulator, that transformer will create a bounding box of all input features.

The output layer can be defined in the Writer, so that shouldn't be too much of a problem.

What you can do as a small trick, is to only write the BoundingBox and to set the source DXF as a template file in the Writer parameters. There you also set both 'Preserve Model Space' and 'Preserve Paper Space' to "Yes". It is advisible (and maybe even necessary) to choose a different filename for the output.