Once you have a nice HTML template built, hack it into a couple of include files that can be reused site-wide. If you are good with Regular Expressions and/or have a consistent layout on your existing site, you can probably fit the new look around your content with a careful search-and-replace. a typical templated page can look like:
<!--#include virtual="/header.htm" -->
// your formatted content here //
<!--#include virtual="/footer.htm" -->
It's bad form to open and close tags in different include files, but in a case like yours you can probably get away with it wihtout confusion.
Personally I'm a big fan of dynamic contextual navigation menus and home-grown CMS solutions; it takes some savvy with databases and server programming, but on a large site it's worthwhile. If you don't have databases handy, there are ways to do it using PHP/ASP and an XML sitemap.
Whatever way you approach this, it's difficult to maintain a large content site with just Dreamweaver. I once inherited a large (2000+ pages) site that was stuck in FrontPage. It was painful; not only using FrontPage, but customizing the navigation in different sections of the site was really arduous as pages were being added daily. I slowly converted that to a dynamic template using the method described above. The sitemap and Page contents were all stored in a DB, and from that point on maintenance was a breeze.
Whether to hire someone or do it yourself depends on your budget, skill, and sense of adventure.
