This project is going to be expensive and complicated and shouldn't run by a novice. Get a network engineer in. But here we go:
Questions that need to be answered:
How are the computers used at the moment? Is there a current internetwork inplace e.g. ethernet cabling connected to switches/routers etc.? What are the computers going to used for?
How much money is there to spent?
Basic solution if no internetwork inplace:
Basicaly to connect 300 machines across 8 floors, you need to wire a ethernet cable from the desk sockets to a wiring closet and connect to patch panels, repeat on every floor. You would then connect switches to the patch panels and link the switches to at least two distribution switches with gigabit trunks and redundancy trunks. Link at least two gigbit trunks from the distribution to the server switch. Connect the server to the switch and setup the switches and the server etc. Then add your WAN to a router connected to the dist switch. Not easy to setup and will be expensive.