I would highly recommend that you reconsider creating something from scratch, if you have the capabilities. There are off the shelf NAS distros like freenas, naslite that make the software side of things easy. The $500 ish NAS devices on the market have some serious drawbacks -
- they benchmark *SLOW*, thats what you get with 300 mhz & 256 mb cache, do some research, you can likely double your speeds with a homebrew setup, not kidding.
- if you have a hardware issue in the future, like the backplane/motherboard on a proprietary box, you may lose all your data, depending on where the raid config is stored
- even if the raid config is stored on the disks, in proprietary format, if the replacement part is not available in a couple years, goodbye data.
- many of these small units do not have redundant fans, if a fan dies, your drives will start dropping like flies, with RAID-5 there is not much of a safety net
- RAID-5 is just not good enough (for me), since I can't afford a backup solution, my new config with 1.5 TBs will be using RAID-6, an extra drive is cheap insurance when it comes to my data, SMART monitors email/SMS me (hopefully) in anticipation of drive failure
- At least one brand (Intel) had a known bug that caused their NAS device to prompt to reinitialize the drives, which caused complete data loss (google it)
For the same money, you can build a *FAR* nicer, faster and safer setup. I have a 10 bay case I got from newegg for $150 or so (CHENBRO SR10769-BK-H), has redundant fans (each bay of 4 has a 120 mm in back and a 80 mm in front, an additional 120mm in the back of the case, and I used 2x 5.25" adapters with triple fans in front for drives 9 & 10). In hindsight I wish I had spent the $$$ on 15 bay SUPERMICRO CSE-933T-R760B for future expandability. I found a mobo with 7 onboard SATA, got a couple 2x SATA PCI cards, and a nice beefy power supply. The mobo has dual gigabit I run bonded/redundant with 802.3ad into a netgear smart switch - another advantage over those prefab NAS's. A $10 IDE-CF adapter and a CF card would be a great root drive, or double it up and run it mirrored if you're paranoid - takes up no bays, power, SATA channels and it's highly reliable, solid state.
I can't stress enough, proper cooling is critical, if the drives run hot their lifespan WILL be shorter, you want as much redundant cooling as you can get, fans die all the time, I tried to buy the highest quality fans I could, and I just replaced one recently. Look at enterprise equipment, theres lots of fans, when you walk into a data center the fan noise can be deafening (ok, well makes it hard to talk at normal levels anyways

).
You will use all the free space you have, and then some, so plan way ahead for upgradability and expansion - thats why I wish I had gotten that 15 bay supermicro... I have 6x 500 gb drives I'm moving to raid-6 1.5 tb units now, and it requires 10 bays just to do the data migration. Thats another thing to keep in mind. When I got my 500s I couldn't believe the space I had, now it's peanuts... in a few years we'll be saying the same thing about 1.5 tb units. Another advantage of running homebrew, at least with Linux + RAID + LVM, you can easily add drives and dynamically grow your raid/filesystem as you need additional storage. Not sure if you can do that with these other devices, either way you'll run out of bays quickly.
eSATA enclosures would be a very nice modular solution as well, add on a PCI card and enclosure as you need space, however all the quality eSATA enclosures with redundant cooling I saw were very expensive, that 15 bay supermicro would be far more cost effective than a couple of those.
Rob