To a large degree, the point of RAID is to not care about drive reliability, trust the process. Also, you seem to conflate RAID with backup ("RAID is not a backup"), you want both. In a NAS, you're probably better off with RAID5 + backup.
In a system that can take a drive failure, the current datahoarder zeitgeist is Manufacturer Recertified (Enterprise) Drives, see ServerPartDeals.com if you're a yank, other countries have their own options.