Hello. I've been working with Wordpress since version 1.5, and have followed all the changes over the years. I'm confident I can fulfill all your requirements as I have been writing/rewriting themes and plugins, as well as making changes to the core code. Coincidentally, I happen to have a copy of the Thesis theme running on my local Wordpress installation.
Regarding PHP, XHTML and CSS, all the sites I've worked on successfully validate as XHTML on delivery (e.g. [login to view URL]).
My time zone is EST, which is an hour behind yours, so I believe we should be able to work out when to call.