You might want to look at Zile or nvi. Both are fairly simple; Zile is Emacs-like, and nvi is the Berkeley vi. Another to look at would be Nano, a simple text editor that is designed to be easy for new users. I don't know how clean the code is on any of these, though.
I would also recommend reading The Craft of Text Editing: Emacs for the Modern World. This book surveys many of the specific problems that have to be dealt with in writing a text editor along with approaches, strategies, and algorithms for solving them. Its content should be relevant and useful even if your editor isn't going to look much like Emacs at all.
