Promise Pegasus, I think you mean. It's a RAID array, not a NAS. More, it doesn't ship sans-drives. If you wanted to reuse your own drives then I'd say go Drobo, though I'm not sure I'd actually recommend a Drobo to anyone who's tech-literate.
Besides, the real benefit of Thunderbolt is for high-throughput data transfers, such as editing raw 4K video. For anything else FW800 will work just fine. In that situation, just grab a multi-bay FW800 enclosure and drop some 2 TB Samsung or WD drives in (as meets your preference, I actually recommend mixing brands). Macsales.com has a pretty good aluminum 4-bay RAID enclosure that will doe RAID5. If you fill it with 2 TB HDDs you'll end with 6 TB of usable storage, which should be enough for most people.