1) Install Wikipedia software locally and use this for any locally created articles
2) The web server running this simply proxies out to en.wikipedia.org for that request if not available in the local version. The easiest way to do this is with Apache + rewrite rules
This means that users can get to articles locally and on wikipedia from the same command
You then need to consider the following
1) The search request needs to go to the local version of wikipedia then the external one and concatinate the results together - a small proxy script should be more than capable of doing this
2) You may want to create a reference table which maps external wiki articles to related internal ones. Again a small script could insert these into the external wikipedia articles during rendering