Have you tried a Framing Shop.... You can chose the frame and have it made to the size you want, and they can provide the mirror. We have a wonderful place close by, and I get all my framing done there. They'll let you bring a sample home to try. Not all are over the top expensive, just shop around. Good luck with your search.

extra fascia depth would be dependent on how much you want to spend on extra labour by not using standard steel fascia (which is only about 185mm wide) with all the standard fixings ..A carpenter or plumber should be able to make a wide substitute out of sheets of ribbed colorbond steel and the rest should be standard timber framing and soffit linings but check wit a carpenter or roofer about what you want to achieve (why does it need to be 40cm wide??) and there may be a very simple alternative available

We finished our antique island bench, which was originally the haberdashery counter from David Jones in Newcastle, with a burnishing oil. The surface proudly bears the markers of it's history as the timber was too soft and thin to strip back any further than we have, but the burnishing process has protected it from every day wear and tear including water spills and the rough and tumble that our four boys dish out to every piece of furniture in the house, it also gives it a beautiful shine. We have finished our dining room table the same way. Kitchen dresser and the sides of the island bench that don't get the same use are finished with beeswax.

Personally if I were to put a runner down the area I'd keep it either plain or a very soft muted colour palate maybe with the grey and natural colours as right now it looks quite classy so you don't want to make it to bold or in your face just soft and calming for the area to enhance it all as one look. I don't really think you need one at all as I love the softness of the area as a whole as it is, sometimes less is more but it once again comes down to personal taste.