I confess, I don’t quite understand what is meant by the ‘automated’ option.
Would people want to purge old articles, even if they’re not visible anymore? Chances are, the moment they’re purged, someone is going to want the details again.
At worst, you could offer an admin purge process, which is probably more in keeping with maintaining the application contained within a PHP / MySQL solution base, rather than having to rely on timed cron jobs, which may or may not be available to some people wishing to use WordPress.
However, speaking as someone who would like to have the ‘expire’ functionality, I wouldn’t want posts automatically purged, unless there was another flag at post creation / edit time to control this behaviour.
As another alternative, an admin page that displays all expired articles and allows you to select those you wish to be purged?