ideally she would be able to edit the text on her own (she is NOT technical)
In my experience, this has been the big sticking point. Many of my clients have expressed the desire to update and make edits to their site after it is created, but that is not easily done if they are not "technical". In any case, if she hires a web designer for a custom design, make sure she let's them know ahead of time that she will be updating herself as it will dictate how the site is designed. For instance, a complicated CSS design would be out of the question.
She really only has two choices for making ongoing edits herself -
1 - She can use web editing software such as Dreamweaver, Visual Studio, etc., but there is a big learning curve. The software can be overwhelming for someone who has a hard time learning such things. Also, there is no perfect html editor so if search engine optimization is important to her she will need to either learn how to go back in and clean up the generated code or come to terms with the fact that the code may not be at its best.
2 - She can work with a CMS (Content Management System), such as Wordpress or Joomla.
Is WordPress.org's tool and plugins really robust enough for this? What are the drawbacks of using plugins from so many different sources when it comes time to upgrade?
Probably the biggest problem would be the shopping cart. Wordpress was created as blogging software not as an eCommerce solution. There is a plugin, but also MANY complaints regarding technical problems. Problem-solving cart problems is not easy and w/free software she would basically be at the mercy of forums to help her work things out. She may find help, she may not. If she does find help, she would still have to apply the fixes suggested which would generally include editing complicated code.
Plugins can be hard to work with. As you mentioned, they come from many different sources and are not guaranteed, so if there is a problem it is not easy to get help. I always read through the reviews looking for issues & complaints before using. That said, installing plugins on wordpress is pretty simple via the admin panel.
If she chooses to go with a CMS system, her site will have a "template" look. As an interior designer the design limitations may be a consideration for her.
The site you have layed-out is not complicated, though the add-ons are not generally included in a standard web design package. The site could easily be created without
the use of a CMS system via a custom web design with the added benefit of more plentiful design options. Have her inquire about the web designer's ongoing maintenance fees. Many times it is more cost effective when taking the time/headache involved in updating your own site into consideration.
Hope that helps!