Large RAID 5 sets with 1 hot spare are almost guaranteed to give you data loss. As a former storage and backup/recovery admin, let me assure you that double-disk failures in RAID 5 sets are a LOT more common than you would think. After 1 drive fails, the rest of the drives get beat up royally while you're reconstructing your raid set and if a drive fails during that rebuild, your data is gone.
I have personally witnessed several RAID 5 sets die and won't ever use them on larger raidsets. As the old saying goes, "been there, done that, didn't like it much". With 1TB SATA drives, you're just asking for trouble. The math is out there in cyberspace if you want to see the limits of errors on drives.
When you go through your design, think carefully about what will happen if one of the raidsets fail - you may be replicating between the buildings, but if it takes days instead of hours to complete the replication when one side dies, you may be setting yourself for a world of hurt. Also factor in how fast you can physically replace a failed drive. If you lose a drive on a Friday night and don't order a replacement until Monday and it doesn't show up until Tuesday or Wednesday, you will have NO spares if you take drive failure during your business day on Monday. I've seen this happen too.
