If you are in the web design and development industry then you probably use an off the shelf content management system such as WordPress or Drupal to develop your content managed websites. These types of software can be very user friendly, are easy to update and often offer a lot of flexibility. The standard versions are also free but then you can pay to add on more plugins and features. Many developers come in to issues when they cannot get the CMS to work they way they need it to for their client and often trying to change the structure of these templates can cause other issues. You may also have an issue with reselling the systems as you may find that you infringe copy write if you try and sell it as your own.
If you have the ability, then you could create your own bespoke CMS system which will give you all of the control and complete flexibility to add functionality when needed. It can be costly to set up initially with the man hours involved, but once you have it there is little running costs apart from updates etc. If you do not feel confident then you could approach a developer who will be happy to develop a CMS system for you and then sell it to you to use.