Thanks for all comments.

Concerning hardware I guess I'll go with big SATA disks (750 GB are
the biggest I've found) directly connected to Promise SATA300
controllers and a wm(4) gigabit nic. I'll see what remaining hardware
my local dealer recommends... ;-)

Then I'll try to setup a raid(4) based software raid on these disks,
should yield about 7 x 750 GB = 5.25 TB at RAID 5 with 8 disks.

I'm not sure about the file system choice: For the users it would be
easiest to have one large partition on that (Do samba/WinDOS and NFS
support such a big partition for network access?).

Does lfs support such big partitions?

Any hints for newfs-ing such a big partition with ffs2/lfs (tuning
parms etc.) then?

I can (offer to) do some testing with this system during installation,
but once in production use it simply should work reliably... ;-)

Any comments/recommendations on my plans?

Markus.