Its certainly a good point about noise; the typical server room environment is not one in which people are expected to work for and extended period and is quite noisy. Even though you would only have one machine, the necessity for small, high rpm fans is likely to lead to the noise having an unpleasant characteristic. this might not be a problem if the server is out of the living environment...
I have no experience with these rack servers, and was wondering what the differences were. Scanning ebay, these servers regularly appear with 2.0GH or more CPUs and several SCSI drives. Are they "old hat"??
Whether they are 'old hat' or not shouldn't be an issue; how well they do the job should be.
The difficulty with 'tower cases', in general, is that you do not know how good or bad the power supply is - this doesn't really apply for server hardware.
The cheap power supplies are failures waiting to happen. They probably won't failed particularly soon if they are rated generously, but run them to their supposed limit and they will give up, either immediately or after a short period. On the other hand, in a 'profesional' system, you can at least be fairly confident that the power supply will meet its ratings. So, I would consider one of several routes:
- over-rate the power supply
- replace supply with a better 'brand'
- buy my tower case from an established server supplier
Note also that once you lose the gui, you can do quite a bit with a low level of hardware resources. The only thing that you have that sounds stressing is the AV stuff (and I don't know how much bandwidth that will be using...) but otherwise quite low end hardware would do it for you. And the AV stuff is primarily a matter of having big fast disks...