Most Wireless routers have a range of up to 300 m assuming that there is a clear line to from the router to the workstation, an ISP with unlimited bandwidth would be advisable, and unless you are prepared to create a domain, you are going to be limited to 252 pcs on the network and it is likely to be pretty slow at the far end!

So the minimum kit you are going to need is a cable /adsl modem router, though you might want to consider an access point half way between the further point and the router, to "boost" the signal

If you are planning on allowing your "customers" to use the network, putting security on the network is going to be pretty pointless.

Do you plan on charging for the service or is this a bonus for your customers?