Audit-proof files in Sharepoint

Hi guys,

we have a special case for a client where i could need your inputs.

Our client uses sharepoint online and for some files (defined folder / defined files) he needs the following:
- 30 years retention
- audit-proof
- unchangeable / undeletable
- Ideally: Digitally signed

I know we could achieve this through the records center or through the Compliance Center etc.

But what is best practice? How would you approach that? Any experience?

What licenses would be needed?

Many thanks!