Managing internal Purchase Order forms/requests in CiviCRM
I have done this a long time ago but it was very specific to the org's workflow. It involved a custom activity type, some autoupdating of a pdf template, and then an internal approval -> email -> offline vendor signature -> email/fax -> inbound civi email processor.

I'm not sure if cases would add anything unless there's more going on too.

You've said wordpress but possibly drupal webform plus this online signatures addon could be useful, i.e. the PO is a webform, and then it gets signed online by the vendor and stored in civi.