How (not?) to create a large, expandable storage/backup server

Backup to tape sucks. That's why we started looking at large NAS devices to be able to back up all of our data to disk. It would be much easier and faster than our current tape jukeboxes that consistantly have hardware issues, or have tapes failing, or run out of room. The problem is, we have a lot of data, and we'd like to be able to keep a few versions of said data. We also have been burned enough times by RAID 5 that we wanted something more redundant. And, to top it all off, we're on a budget. So here's a basic rundown of our requirements: