New DataHoarder setup advice

Hi everyone, I am planning my storage setup and would like to hear your advices/ideas/suggestions/criticisms.

The use case: AI model storage (diffusion, LLMs maybe other models in the future), local code repo, some scraping and personal data potentially used for training.

Requirements:

• 24 TB expendable to 48 TB or more in the future.
• no need for it to be online all the time (happy to turn it on/off when downloading/retrieving data).
• some form of backup/redundancy in case of drive failure/lost sectors.
• cost effective.

Initial plan:

• second hand server rack (Dell 730?) with 8x3.5 bay
• TrueNAS SCALE in RAIDZ2
• 4x8TB HDD 5400 RPM (is is worth getting the MTBF 1M or 2M?)

Current cost estimate: server ~300 USD, drives 4*150 = 600 USD = ~900 USD (note that I do not live in the US, but I translated the prices I can reach to USD for convenience)

Any feedback?