How to add a user-friendly option to link to nodes from body text or text-field?
Sounds like you're looking for the LinkIt module.


  Linkit provides an easy interface for internal and external linking.
  Linkit links to nodes, users, managed files, terms and have basic
  support for all entities by default, using an autocomplete field. In
  7.x-2.4 there is also support for fields (text, textarea, and link).


If you have pretty paths I'd also suggest combining it with Pathologic, that way your links will have their path aliases applied rather than their direct links (i.e. node/123 will become about-us)