The corona varies dramatically in brightness from the inner portion near the sun to the outer portion a few solar radii away. The innermost portion is the brightest; at ISO 100 and f5.6, an exposure of ~1/125 second will capture it. So it's roughly the same brightness as the full moon. The outer corona will require much longer exposures of perhaps a half second, but note that during this time the sun will move noticeably with a long lens if you're not using a tracking mount, so some blurring will likely occur. Overall, practicing on the crescent moon will give you a pretty good idea of the dynamic range of the corona.
The diamond ring is dangerous to look at directly, as is any unobscured part of the sun's disk; you should look away at first glimpse, or earlier.
There's lots more detail scattered around the web, which you've probably found by now. A good overview is available at http://www.mreclipse.com/Totality2/TotalityCh12-3.html#SEExpoTab