The myth that there is something unique in having a "signed" case is regularly perpetuated on eBay and such, but it's bunkum. Now, if you have one signed by Woz in felt marker, that is a little different.

Another vote for Retrobrite. Retrobrite results in a much, much, much more attractive overall effect than painting. This is a good introduction:

. It is the only way to get a keyboard/ cables/ peripherals that match, and that is essential if what you want is a machine fit to display. The only real exception would be if what you wanted to display was not so much your machine as your paint job, but if you look around for images, you'll see that even the best of those mods are never typically displayed with the matching peripherals, because that way they look like crap.
I would just glue the crack with superglue or the like. If you have more serious damage, e.g., if the case has been hacked beyond recognition, then sure, it could be poly-bond and paint time, but that will never look as good as a properly restored case.