Have you considered Windows 2003 Small Business Server? It's a proven platform with plenty of information on the internet, and obviously good compatibility in terms of drivers, software, etc.
For a long time I looked at network attached storage, Linux storage servers, etc. Nothing ever seemed ideal, at least for the price. Eventually I came across an HP server for $899CAN that included SBS. I don't have the storage requirements you do, but I picked up a couple cheap 320Gb drives and slapped them in, and have been running that setup for the last couple months. It has 4 SATA drive bays (not hotswap, but easy to change), so you'd need to add some sort of controller and use an external HD enclosure, but it may be a good starting point.
Proliant ML110 G4
Dual-core Xeon, 1Gb ram with room to expand, built-in RAID 0,1 for the 4 SATA drive bays, 2 5.25 bays, gigabit lan, PCI and PCI Express, etc.
There were only two things that annoyed me... First that it took about a month to arrive after I ordered it. Second that there is no floppy controller at all (still needed for installing RAID controller drivers during Windows installation). I got around the second issue easily by finding a USB floppy drive, but it was still annoying.
Now I have my own little domain at home, along with an Exchange server. I might just have to get myselft a static IP...