One common method of managing insertion and deletion into the buffer is to use the BufferGapMethod. Other methods include using LinkedListsofLines (used by IbmEditorFamily editors like XEDIT), and LongStrings? (not used beyond the simplest editors).
CraigFinseth wrote a great book on DesigningTextEditors and implementing TextEditors that goes over these methods as well as the Pros and Cons of each.