I am not bragging, I'm remembering some bad days.
The hardware needed:
* An Internal Wireless LAN 802.11g PCI Card adapter.
* A coax network, leading to different antennas in your 300-400 m campus area, delivering to your wireless card.
* An Ethernet card (to your xDSL or cable modem).
or * An ISDN card, if your ISP supports it.
or * A dial-up modem. That time I did it with 4 dial-up modems, and I won't get into that, to keep your sanity graph from declining.
The software:
* your Linux system.
* set a DHCP, accepting connections on the wireless card network only.
* set DNS server.
* set routing and enable NAT.
For the last point, search "Google dot com slash Linux" for "How To Masquerade On Linux
". Nowadays I recommend 'iptables' on a 2.6 kernel.
Finally, if I were you, I'd buy a $40 wireless router/access point, from somewhere. Most consumer wireless routers today are a stripped down PC, running embedded Linux or someBSD in their core, with the above settings.
However, it's a good learning project.
Good luck.