What is more expensive for your organization: time or money? In general, your options that cost less take more time to setup, and vice versa.
It seems like cheap is more important, so I would roughly do:
- SSG like Hugo or MkDocs
- store the content in S3
- serve with a CDN like Fastly or CloudFront
- authentication via VCL or a Lambda using OAuth
The stuff that rolls off this man really does defy logic.