Wordpress is a pretty simple platform to use once you get to know your way around. It is pretty easy to adjust from an SEO point of view depending on how meticulous you want to be.
Plugins install into Wordpress rather than the theme, so you will be fine on this front. Some themes come packaged with some plugins pre-installed which is where your confusion might be. The preinstalled options are usually optional allowing you to use a different plugin if you don't like the theme one.
WP is pretty easy to work with from scratch. You create a database, assign a database admin with full privledges making note of the login/password. Download the latest WP version, upload to the root directory, unzip and then run the WP install wizard. Before you know it you'll be done.
Genesis and Thesis are both considered frameworks which allow custom websites to be built on top of them using shortcuts. They are more complex in the initial stages but developers who use wordpress a lot come to appreciate the flexibility they offer. There are those who build themes for these frameworks too (as opposed to Wordpress themes). So it can get a little confusing.
One thing I would HIGHLY recommend is to build a child theme for whatever WP theme you use/buy. They are simple but I would say essential and will save you lots of work. The reason for them is that it keeps your theme and any customisations you make separate. The biggest reason is that in the future if the theme receives an upgrade and you've made some customisations to the CSS etc, you may lose all that work. A child theme protects those changes and allows you to safely and happily update your purchased theme at will.
Video support is something that is built in to WP. Most of the major formats you'll be using will be supported. The way the video is triggered and presented may vary using themes. For example some themes may present video in a pop-up lightbox. [/QUOTE]
I've not had any experience with Canvas but have used U-design. It is very feature-rich and has good documentation. It can be overwhelming because of the amount of features but the documentation usually nails how to set things up.
I always recommend going for elite authors and those who support their themes, U-design certainly fits this. The others high on this list would be worth looking at too.