Among the finest to create this quite simple website. Essentially you will find multiple pages A, B, C... and every page has item A1, A2.. or B2, B2... These products follow fixed HTML template (table,image) and just specific content fields (title, body text..) could be alternation in the rear-finish. After sales customers can also add new pages or new products each page.

Does anybody know some wordpress plugin for either Joomla, Drupal or Wordpress that may perform the simple purpose above? Thanks

In Drupal you may create nodes with content type "page" (standard built-in) after which make use of a module like "Views" to show this content. Should you browse around the Drupal site you may even look for a module that matches excatly your requirements.

In WordPress you could make your personal page templates, in which you define which parts are editable for that after sales user and that are frozen. No requirement for a wordpress plugin.