We are currently looking at a re-design for a static site and intend to use Drupal cms.
The worry I have is that the site is well established with static url's widget-one.html / widget-two.html style file structure that ranks really well.
My worry is that were we to revert to cms the urls's become widget-one, widget-two with no html suffix. Does this affect the url to the extent that Google will see a "new" file and consider anything else a duplicate copy? Or is it the case that the file name (if the words and hyphens remain exactly the same) will not be disregarded simply because the .html has been dropped?
You don't really need "add-ons" with Apache since it normally ships with mod_rewrite and if it's not already enabled, you just need to activate it.
You are not "converting" pages to HTML on publishing. The pages are HTML pages no matter what, the question is at what address they appear. mod_rewrite simply translates one address to another.
Think of having call forwarding on your phone. If you have your home phone forward to your cell phone, I as the caller don't know that. I just dial the old number (in this case "widget.hmtl") and it takes me to the new number (in this case "widget").
Any CMS will work with this process (as will any static page). It's like asking "what phone should I use with call forwarding?" It doesn't really matter. It's happening at the switching level, not the phone level. So to this is at the server level, not the CMS level.
So, I like Drupal, but there are many others and some of those might suit your needs better.