Congratulations!
Good choice on the raidZ2/Z3. With standard raidZ, you're pretty much asking to lose an array. You can easily have a second drive failure just recovering your array. If you don't have a drive set up to automatically recover the array, you're just asking for failure. You have to at the very least keep a spare or 3 on hand. If someone didn't have enough disks for RaidZ2/Z3, I'd suggest some form of mirroring, or constant backups. In any case, I'd still suggest keeping a drive or two in the box set up as a spare though. Just less annoying, and less likely to lose an array. Also still need backups, because shit happens.
The drive choice is a tossup. The reduced number of the large drives should give you a lower chance of failure and increased options for expansion...but the used drives are likely less likely to fail right away, due to the "bathtub" curve. New drives tend to fail. I'd probably go with the used drives, due to the lower costs. But that also has slightly more power usage.