How can I decide if I should roll my own software or use a packaged CMS (and appropriate plugins)?

I'm a software engineer with little web experience currently in the process of wanting to actually using my web host and domain name for something other than a file dumping ground for files that I need to give other people access to. My plan is to somehow deploy a personal/professional website and blog, either using FOSS off-the-shelf products, custom software, or off-the-shelf components with custom modules/plugins.

My requirements are:

1. Semantic HTML (well formed, proper use of attributes, microformats where appropriate, use of CSS for style and HTML for markup)
2. Validating HTML and CSS (XHTML, HTML4, HTML5 all acceptable - as long as it validates out of the box)
3. Support for a blog
4. Support for editing and managing my resume in HTML form as well managing uploaded files in PDF, ODT, and DOC formats that I make available for download
5. Written in a language that I know and my host supports - PHP preferred since that's easiest for my to host and I also am familiar with it
6. Easy to obtain or write a themes for
7. Support for static pages that change infrequently (about me), static pages that change sometimes (project lists), dynamic pages (blog, comments), and forms (contact me)
8. Plays nicely with other tools in the same environment - I use Trac on the same host for project management and might deploy other software such as a personal MediaWiki deployment
9. Easy to integrate with other social media (Twitter? Facebook? Pandora?) - not require, but would be nice
10. Generation and maintenance of the sitemap and/or robots.txt

My questions:

• What, if any, FOSS solution would be appropriate for me? I've toyed around with Drupal and WordPress in the past - WordPress struck me as a solid tool, but at the time was very blog-focused (I understand it's changed more recently to be more CMS-like).
• What free (but not open source) solutions might be appropriate for me? I would prefer to have full access to the source, but it's not a requirement.
• If I am going to roll my own, where should I begin? I would prefer not to roll my own - my software engineering background tells me that my requirements should be common and I should be able to find some existing solution(s) that I can combine to make this work.

Thanks.