Depending on how complex you need it to be you have several options... WordPress is amazingly easy to use for a start. If you get a theme which is solidly built, you can alter the css and make it suite your needs very easily!
For simple pages where your client would simply like to log on and amend text and images etc, you can look into services like Cushy CMS which is extremely simple to use, but relies on 3rd party servers, or look into CMS from Scratch, which is very easy to set-up and doesn't require any database resources. I've written a little article about CMS from Scratch on my blog if you want to have a look. Though I will mention that the last version I've used that is relatively bug free is 1.2.1, not 1.9.2 or 1.9.3 so I'd suggest you look at that earlier version.
Also, I agree with Dave regarding e-commerce - Magento seems very capable and is a rising star in that category.