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The default font is set differently from everything else you're trying to do. To set a default font, go to Text menu > Text and Font. If you have a really lot of fonts, it might take a moment or 2 for the list to populate. Then choose your font, choose the font Size, and click the Set as Default button.
Note that this only sets the default for the new text that you type. If you open a file which is using a different font, you will see whatever font is specified in that file. Also, I feel like I should say that for me (Windows 7) my default font often changes by itself. I've never figured out what happens that it changes. And I have not heard the same complaint from anyone else. But for me, it changes by itself, and fairly often (considering it really should not change without my input).
Anyway, to continue with your questions.... When you say you set your rulers to mm, how exactly are you doing that? The correct way is in Document Properties > Page tab > Display Units. This is at the very top of that page. About halfway down the page is another place to change units, but that is for the page size. Most of the time, I guess users want to change them both. But I only mention it, because for some reason, a lot of people seem to notice the units next to the page size, and don't notice the Display Units at the top.
When you click Save As, you should go to C:/username/AppData/Roaming/Inkscape/templates. If templates folder doesn't exist, you can make one. By using this user directory, your custom default doc will be available to you, even after upgrading Inkscape. If you put it in Program Files directory, you'll have to re-create it after every upgrade.
Also, after navigating to templates folder, there might not already be a default.svg file there. So in the Save As dialog, instead of clicking on default.svg, you'll have to type the filename yourself. Type "default.svg" (without the quotes) in the Filename field, and in the Save as Type field (which is really a dropdown menu) choose Inkscape SVG (.svg). Actually Inkscape SVG (.svg) is probably pre-selected for you, but not always.
And finally, here's a tutorial with even more details, in case you might find more helpful info:
http://libregraphicsworld.org/blog/entry/using-templates-in-inkscape
