What sort of requirements do you have for a document management system? I'm
not an expert in this area, but it seems that SharePoint would do the job
regardless of the type of document.

I've already created a Document Center Site and associated work flows which seem
to work well with MS Office based documents. If I store an XML document with
external links to XSL, XSD, or other XML documents, I cannot open them up from
SharePoint (even if these external documents are stored in SharePoint and
appropriately referenced).

You can store XML files in SP..........