1) Build all machines yourself. I have built over 20 myself. I just built this PC that I am now using last Sunday. I recommend your server to have a dual-core processor due to the mostly multi-thraded requests it will be processing. Look at the Pentium D or Athlon64 X2 processors for fast performance. I personally use the Pentium D 830.
www.ncix.com for all your parts.
2) Build all yourself.
3) RAID5 since SCSI just is too expensive. Also, when you create a RAID5 array, use the latest generation hard drives to ensure performance. Seagate and Western Digital already offer 2nd-generation SATA-2 based drives with 3GB/s throughput. Blistering fast. Look for Western Digital models more since some offer a 16MB cache, as opposed to 8MB. Trust me on this one - I got one for myself.
4) Dunno
5) DVD-RW since they have realy become cheap these days for what they do. I recommend the brands NEC, Pioneer and Plextor personally.
6) Windows 2000 Server and Windows 2000 Pro would be the easiest and cheapest solution. However, you can run a Windows 2003 Server with Windows XP Professional clients. If you build workstations with 64-bit capable processors (i.e. Pentium 4 6XX-Series, 8XX-Series, AMD Athlon64 Series), you can run Windows XP 64-bit Edition).
7) You might consider Network Attached Storage instead of tape-backup since that's an aging technology anyway.
If you're interested in how much each thing will cost you, drop me a line with some details. I am certain IBM or HP would rip you off.