TinyMCE is provided with the default MODx installation. It is applied to resource content, RichText type TVs, and optionally for chunks.
You can create "roles" that control what your editor user can do; after setting up the role with the permissions you want, just assign the user to the role.
You would probably find it simpler to use the front-end editing supplied by QuickManager. The editor logs in to the Manager (you can configure the editor user to be sent to a specific front-end page on login; it’s in the User tab when creating/editing a manager user). The QM menu is hidden at the top of the page, and drops down when the mouse is passed over it. QM is highly configurable; go to Elements->Manage Elements and the Plugins tab, open QM and go to its Configuration tab. You can disable it for pages you don’t want the editor to mess with, you can remove items from the menu or add your own custom items. For example, if you have a blog editor you could add a link to an unpublished blog management page (manager users can view unpublished pages, and even this is configurable using roles) that uses Ditto to provide links to all the existing blog pages. A manager can be denied/allowed access to the Manager itself in his User tab.
Since QM allows editing of the TVs attached to the page’s template, your editor can edit the TVs for the page. I believe the latest version of QM even adds buttons on the page itself at each block’s location. Using TVs instead of chunks for blocks of content is perfect for this, since each block can be customized for each page, just use a default set when the TV is created, or inherit a parent’s TV content if the default was set to @INHERIT. You really can’t make use of half of the power of MODx until you get a start with TVs.
http://www.sottwell.com/how-template-variables-work.html