Designing a semantic search

I have just started freelancing on a complex project, a brand website integrating product, editorial and community content. In the past two years the design team has focused almost solely on UI, and provided the client with page-templates and specifications that could be customised to fit into the platform architecture, once back-end development starts.

The team has been working in two week sprints, shipping essentially modules, pages and micro-interactions at every sprint. There's no clear commitment on content architecture (TBD by client) and there's not an up-to-date site map for reference.

Now, a month before the project comes to an end, I've been asked to design a semantic search, UX and logic, that could allow users to find all - loosely defined - content in the site.

It looks like the client has a few issues committing to specific site architecture and content structure, but nonetheless is expecting this work package to be provided by us.

I am evaluating how to go about this task, without putting too much pressure on the client or trying to backtrack when and how certain design decisions were made - this could take weeks. Any hints?