Multiple articles with individual entries
There are always so many ways to configure a CMS like Craft. Personally I would make:


"Articles" a Channel
"Main Sections" a Structure


Within Articles, you could have a Matrix field, which would in turn contain an Entries custom field, allowing your client to select Main Sections and sort them as content-blocks, using the drag-n-drop features of the Matrix custom field.

From the client's perspective, they would see something like this:



Let us know if that helps, I'm sure others will offer alternatives :)