DIY NAS: Usage intentions are clear, but unsure of build approach

[Enquiries]

I'm looking to build a modest home NAS for me and my parents (two households). I know what I want to achieve in terms of usage but have some questions regarding hardware. On the software side of things, however, I'm looking forward to learning more and I expect to spend a lot of time tinkering and optimising. Some context:

COUNTRY: Australia
BUDGET: $500-600 AUD
STORAGE: 8 TB
GOALS:
- General storage for documents and media collections.
- Self-hosting (BitWarden, Photoprism).
- Jellyfin/Plex media server with 1080p transcoding capability on at least two devices. Power-efficient and relatively quiet.

QUESTIONS
- Is this achievable with my budget?
- Repurpose old PCs or build from scratch?
- Separate device for media server? (e.g mini PC)
- Is a dedicated HDD for redundancy necessary?
- RAM - 8 GB or 16 GB?

Thanks in advance.