Yes, Google's Search Appliance (GSA) could be used, I have seen it used with limited success. The main problem was how to respect access control on documents: either you index them or you don't, and if you index them with GSA, sensitive data may show up in search results. Also, we had a lot of trouble "taming" GSA: it would regularly take down servers that were dimensioned for light loads.
I would suggest using Alfresco http://www.alfresco.com/ as a CIFS (Common Internet File System) or WebDav store for all those documents. This would give you the simplicity of a shared folder and the opportunity to enrich the documents with searchable metadata such as tags, etc. Each folder (or any item, in fact) could have the correct access control that would be respected by the search engine, Lucene. http://lucene.apache.org/java/docs/
Alfresco comes in both Enterprise and Community Edition, it's very easy to try out -- even our non-techie project manager could install it on his PC within 10 minutes. Try that with Documentum, FileNet or IBM DB2 Content Manager!