Just a brief note to comment there's a new minor release of model_auto_completer:
http://agilewebdevelopment.com/plugins/model_auto_completer
What's new:
A wrapper of :after_update_element is provided, which among other stuff receives the selected model ID. For example, using this callback you could send the model ID back to the server via Ajax to update the page with model attributes or whatever.
-- fxn