My though was to use hardware raid, and just set that up for the 2 hdds, then boot off an ssd with Debian (very familiar, and use it for current server which has 30+ docker containers. Basically I like and am good at docker so would like to stick to Debian+docker. But if hardware raid isn't the best option for HDDs now a days, I'll learn the better thing)
Which drives? Renewed or refurb are half the cost, so should I buy extra used ones, and just be ready to swap when the fail?
You don't want hardware raid. Some options you can research:
Mdadm - Linux software raid
ZFS - Combo raid and filesystem
Btrfs - A filesystem that can also do raid things
Some OS options to consider:
Debian - good if you want to learn to do everything yourself
Truenas Scale - Comercial NAS OS. I bit of work to get started, but very stable once going.
Unraid - Enthusiast focused NAS OS. Not as stable as Truenas, but easier to get started and a lot of community support.
There are probably other software/OS's to consider, but those are the ones I have any experience with. I personally use ZFS on Truenas with a lot of help from this YouTube channel. https://youtube.com/@lawrencesystems?si=O1Z4BuEjogjdsslF
Ditto on hardware raid. Adding a hardware controller just inserts a potentially catastrophic point of failure. With software raid and raid-likes, you can probably recover/rebuild, and it's not like the overhead is the big burden it was back in the 90s.
I got a server from ewaste because the RAID card did fail and having SAS drives they couldn't even pull data from it with anything else. It was the domain controller and NAS so as you can imagine, very disruptive to the business. As they should they had an offsite backup of the system and so we just restored onto a gaming PC as a temporary solution until we moved them to M365 instead.
I just use software RAID on it now and so far so good for about 180 days.
This all happened two weeks before I started, so I don't know the exact details. If it was set up the way I think it was, I'd say yes, the DC was in it's own VM and then a separate VM would've been used as a NAS. Of course being hardware RAID the whole host server went down when that card failed.
They probably didn't have a second DC set up due to the DEFCON 5 levels of "We can't work!"
They were ultimately planning on going to the cloud anyway from what I heard and that catastrophe just accelerated that plan ahead
Truenas Scale - Comercial NAS OS. I bit of work to get started, but very stable once going.
Unraid - Enthusiast focused NAS OS. Not as stable as Truenas, but easier to get started and a lot of community support.
Since OP wants to use Docker i would not recommend either. Trunas scale does not support it usefully and the implementation in Unraid is also weird. Also the main benefit of unraid is the mixing of drives, OP wants to raid.
I am aware of vdev expansion since i am following it closely but not heard about docker support, thanks for that new, i will read into it. Would be actually a game changer for a project i am planning.