Because of the flooding in Thailand, hard drives are not cheap at all at the moment: they more than doubled in price, at some vendors even tripled.
And it is expected to stay that way at least until march 2012.
(Statistics from a huge collection of Dutch web shops for the Samsung Ecogreen F4 2TB drive. blue is average, green is minimum price)
That said, we have recently purchased the QNAP TS-809U RP, an 8-drive, rack-mounted, very configurable storage solution with redundant power, dual network and many data redundancy options. Currently with 4x 2TB and one spare disk, we have 6 TB of effective storage and one disk may fail. Later this can be expanded to 8x 3TB where two drives may fail without data loss.
It can do 100 MB/s over a gigabit network, even faster if you use link aggregation on the macs and the QNAP (we have a 100 Mbit network, so it's a tenth of that here). This is regardless of protocol by the way: AFP or SMB don't make a difference in our case.
A history of downloads should be doable, but that will be tricky to set up. For example, you can have the SMB-server on the network drive log activity to a file, even to multiple files specific to one user. This will require knowledge of the file access protocol (SMB) and linux, and you need a NAS box with full access.
See here for a glimpse of the complexity: http://moiristo.wordpress.com/2009/08/10/samba-logging-user-activity/