How to Build WP Site with Hierarchical Content and Using Custom Design?

A client asked me to redesign her web site, built several years ago in WP by another developer. Although I’ve never worked with WP before, I’m pretty comfortable with html, css, and php, and I more or less understand how WP stores content and dynamically builds pages. I have a few questions, though, and am hoping the WP community can point me in the right direction.

My client’s site has about 75 pages. There are about 25 that are static (i.e. the content changes infrequently if at all; things like “about us” and “faqs”) and there are about 50 pages that are more “blog-like”, except that instead of posts, the content contains directory-type info (e.g. 12 DJs in the area) or event-related info (e.g. upcoming shows at local theaters). Both of these categories contain many sub (and sometimes sub-sub) categories (e.g. medical services > pediatric > kid allergy specialists) and the content updates fairly frequently.

I understand the difference in WP between “pages” and “posts”. But I need to find out the best way to structure the static content. Should I just set up a parent/child hierarchy of pages, changing the permalinks to something that makes sense? Or is it better / easier to just build the static pages *outside* WP and somehow link to them from the common navigation?

As a web designer, I want to “wow” my client with a great design. While there are loads of wonderful WP themes available, I really need to create something unique. But I’m wary of breaking something, so what’s the best way to take an existing theme and just tweak it enough to make it look a little different?

Finally, other than mounting a massive “copy and paste” effort when the new site is built, is there a way to transfer content from the original site to the new one?