Rosewill does some nice affordable bare-bone external units. So does SANS Digital.
I would definitely test what you're planning with a smaller incarnation before you committed whole hog to a big installation.
Might also want to check some of the forums (FreeNAS etc.) to see if anybody else has done something like what you're planning. Or tried and failed. Reason I'm saying is that there is a good chance there's a breaking point somewhere once you go over a certain storage capacity and start running into reliability issues with off the shelf open software.
Maybe somebody knows for sure one way or the other?
From everything I've read so far, the thing that is sticking out the most for me is the hard drives. I have a bunch of consumer grade drives that i basically got on sale the past year or two. But they are not ideal for a freenas server, although most say that it works fine. Really, the best way to go is SAS drives. So based on this, I'm going to try a 4-bay smaller implementation of what i originally planned and test it a little bit before going huge and deciding to get sas drives or something.
There are a lot of hobby builds out there in the htpc community like xbmc folk and stuff. So many of them use the Norco 24 drive racks, so they are dealing with tons of storage. it sounds like if I'm careful and follow the guy's advice in that freenas guide above, I should be ok, but it will take some studying.
Another easier option I'm considering toying with is the original "soft" solution. That is, using a normal windows 7 OS or windows server, and using the software tools to deal with the drive pooling like snapraid. The advantage there supposedly is an easier and more flexible setup, like, I can pull drives out with less headache and add drives, etc. The disadvantage is that there will likely be some performance hit. And maybe a reliability hit also, i mean, there must be a reason why the enterprise users don't do that kind of stuff.