If You derive Your View from CEditView, You will have all features for work with text. To read from or save in a text file, You can use SerializeRaw( CArchive& ar ) member - function (it should be in Serialize() - function of Your CDocument - derived class).
To right work with File extensions, You can override OnFileOpen() - function of CDocManager (make Your own class, for example CMyDocManager, and replace old CDocManager with it).