Don't confuse SAN with NAS. A SAN is generally fully fibre channel SCSI using fibre channel switches, I don't know what amount of data you are talking about, but the growth should be pretty hefty if you want to consider SAN. (you can later on add cheap SATA disk cabinets, but we are talking pretty hefty storage here, we just bought a new SAN at work with is now "just" 8TB, but can grow to 173TB, servicing 128 servers.
A SAN is by standard fault taulerant, runs RAID 5 including hot spares. All fibre switches/interface cards are redundant (dual).
NAS is nice and all, but I would still go for DAS (Direct-Attached-Storage) if a SAN is out of your league.