I'd recommend BTRFS in RAID1 over hardware or mdadm raid. You get FS snapshotting as a feature, which would be nice before running a system update.
For disk drives, I'd recommend new if you can afford them. You should look into shucking: It's where you buy an external drive and then remove (shuck) the HDD from inside. You can get enterprise grade disks for cheaper than buying that same disk on its own. The website https://shucks.top tracks the price of various disk drives, letting you know when there are good deals.
possiblylinux127