I prefer recertified ones if they’re significantly cheaper, but that’s up to you. Recertified will likely fail faster but when they’re close to ~60% of the cost it makes sense to gamble.

As for which RAID that is up to you and how you’re setting up your array. If you’re running ZFS then mirrored pairs are somewhat flexible since you can add a pair whenever you want of any size disks, but they will cost you 50% of your disk space in redundancy. For RAID5/6 you want the disk sizes to match and for ZFS you won’t be able to add any disks to a RAID5/6 array for about a year - the code that adds that feature is coming in the next release which will take about a year.