I would definitely go for Red or other RAID/NAS type drives, because these are the only ones to have Time-Limited Error Recovery (TLER). Without this, your disk will hang for indefinite amounts of time when an error occurs. That means that a single sector read error can cause the entire disk to be thrown out of your RAID array, instead of letting the RAID system recover from the error gracefully.
I ran a 6x2TB raidz2 pool for years built with Seagate greens pulled from Craigslist-bought USB external drives. FreeBSD and/or ZFS were always dribbling stuff to disk (logs, metadata updates, etc.), so the drives never spun down. These were using the native motherboard SATA ports.
ZFS is very reliable. Sometimes a scrub would find a problem and fix it. I'd then offline the device causing problems, assuming the sectors were getting "soft", and then I'd run a destructive read/write badblocks on it to flush out bad sectors, then reintroduce it to the pool. The drive would be once again be solid for a good long while.
I'm anxiously waiting for the day inline encryption makes it to FreeBSD's ZFS, as the standard GELI+ZFS method has always seemed a bit clunky to me.
Greens would work, but are limited to 5400RPM so your data transfer rate will take a hit with that.
Reds are WD's "NAS" drives, with some advanced features, but the Segate ST2000DM001/ST3000DM001 have worked very well for me with multiple years of almost constant data access.
Sidenote: the upgrade from 4x2TB to 4x3TB was extremely simple, I just failed my drives over one at a time, so as long as you do everything at once you can expand everything really easily with ZFS.
The seagates worked well for me for a long time when they were directly attached to SATA ports, until I got a chassis with a SAS backplane. Their firmware had some bugs which would stall all transfers on the whole backplane (not just the one disk) for about 30 seconds when there was a lot of activity.
I switched to WD REDs after that, haven't had any issues since.
Can we use the absolute cheapest Western Digital Green drives, with limited lifespans? Does ZFS spin down drives when not in use?
Actually, how long do the cheapest drives last when spinning 24x7?
I ask because Best Buy has 5TB Western Digital drives for $110 today: http://www.bestbuy.com/site/wd-my-book-5tb-external-usb-3-0-...
But these consumer drives never list reliability figures, unlike data center drives.