You can set-up Wordpress as a CMS. I'm using it on half a dozen sites now. I use the Semiologic Pro theme which includes a lot of great plugins, layouts, templates, basic skins bundled together. Making quick changes to layout/style is easy.
For the CMS part, there are a couple of plugins in Semiologic that really make this work: Static Front Page, and Silo Site that arranges your Pages (not Posts) into a SEO friendly structure. Add this to the Widgets and you can completely control the menu (disallowing certain pages, etc.), what appears in the side bar(s), etc. There's also a top nav and footer nav that are configurable (as to which links get placed there).
URL's - you can specify the name per page, with mod_rewrite you can make them the same as what you have now (adding .html or whatever)
Editing is WYSIWYG, even images can be uploaded from there. You don't have to use the blog part, although you could use it as a news page.
There are a number of layouts available within Semiologic - then you use one of the skins within the Semiologic theme to set your look and feel, then you modify from there with a custom.php and custom.css. You can override certain things on your Pages using custom.php (like the comments, trackback stuff that doesn't really need to be there). You can even integrate other stuff into your own templates if you need to extend the functionality. It sounds complicated, but it's not really.
Another plugin I use is Adspaces, which let's you configure a default set of ads (adsense, or whatever) and apply that to pages on the site - makes it easy to manage. (your client may not use this, but could use it to promote other areas of their site, since it accepts html) You can specify it on a page level, too.
It's pretty flexible and there's more to it, but these are the basics for CMS. Hope this gives you some idea. :)