I'm using the Deskstar NAS model you plan to buy in a low-performance fileserver. Bought the drives last year, in early December. None have failed so far and no one complains about their speed.
Regarding their use in a RAID 5 array, like Merc pointed out, I wouldn't use those in a large RAID 5. They are rated for one unrecoverable error per 100 Tb written (1 per 10^14), so one error per ~12.5TB. You plan for an 18TB RAID 5... so you'll get at least one error per rebuild. Garantied by the specifications of the drive.
For a RAID 5 that large, I would at least use drives rated for one unrecoverable error per 10^15, but all those are quite a bit more expensive. The cheaper one is the
WD Red Pro. I've never used it. The Seagate ST6000VN0001 (Enterprise NAS 6TB) cost ~300U$, but isn't stock at New Egg at the moment.