Also do you plan to cache some documents that are often searched? I assume there's a 20% of documents that's searched 80% of the time in each organisation and part of the 20% rapidly changes (so there's a decline of the 20% with information constantly incoming but for simplicity let's leave it at that).
Is there a possibility you will perform "auto tagging" on documents? (by channels on slack, folders on drive, people involved, project names, basically any meta info that might allow to narrow down the serarch?)
Also, I hope you're gonna have a self-hosted version, since I believe that may be a make-or-break for many orgs.
Besides all this, awesome, you found a dope problem to solve, this might be huge!
On the technical side, we currently see this as something that can index on-the-fly, and maintain an internal representation of all the people who have the authorization needed to see the document.
I really like the idea of "auto-tagging". There's already research done on algorithms that generate tags from bodies of text. So it shouldn't be too difficult to integrate that into indexing.