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?
No hardware RAID. Use zfs, if you can. Mirror the boot SSD. I would use a stripe over mirror and 4 HDDs. Two drives are not enough redundancy.
Use enterprise or nearline drives, if you can.
Debian is great, you can install Proxmox on top of it, but from the sound of it plain Debian would work for you.