I think normal is about right... for normal

There are two harder modes and only one easier mode, so normal is below average difficulty.
As for mods (maps etc etc), there's a lengthy discussion here. http://www.gamesas.com/topic/1386585-mod-tools-for-pc-version-should-come-to-dishonored/
No tools planned for now, and that means no developer-sponsored Steam Workshop. But that doesn't mean there won't be unofficial maps and other mods at fansites. If people really want to they can figure out how.