Comment 2 for bug 1362061

I believe that most of what you want here is already available within existing inkscape. When you start Inkscape by itself, with no document specified, you are actually loading a default document, whose properties you can edit if you want to. On Windows, this default document is located at:
C:\Program Files\Inkscape\share\templates\default.svg.
At this location there are also other templates such as default_mm.svg, etc.

In my case I normally get a default document with px as the unit. If I edit default.svg, or overwrite it with some other file, I can get mm as the default unit on startup. As far as I know, I think the same applies to all the settings in the document properties dialog.