Note that Wifi has terrible, terrible throughput. Streaming video and copying large amounts of data over it is asking for pain. Maybe 802.11N has enough bandwidth to not suck so badly, but that would require replacing the router and all access cards (assuming they are G currently)..
1) That depends; are you wanting to have it stream media files, or just act as a file repository? Are you wanting to set this up in the common area as a media viewing platform?
2) If you have a linux-based router, you can set up some iptables (IIRC) rules to do that. Don't ask me what the rules would be specifically, though.
3) Find a deal tracker site for your country, and watch it until hdd sales occur.
4) Depends on the motherboard you use; many of them support some versions of RAID 0/1/5 these days. Make sure not to fall into the fallacy of RAID 0.
With lots of drives, you should be able to do RAID 5 without much hassle.
5) You need an order of magnitude less processing power to serve content than to create it. It might be better to get a $50 Athlon than a Celeron, though. You can always get 1 GB of ram, and add more later if you need it (make sure to by 1 GB stick, not some combination of small sticks that add up to 1 GB).
edit) If you go Linux, its probably a better idea to set up Samba than VNC for copying files to the machine. You could set up a /home/share folder on the machine, then add this as a network folder to everyones desktop/my documents folder.