Looking through ebay I came across this
Aironet Arlan 630-2400 2.4 GHz Ethernet Access would something like that be suitable. It has a 10base2 connector meaning it would be easier to connect to the existing backbone. However information on that device is sparse and how would I go about setting it up? Would it work the range I require/what WEP does it have? Does it even support 802.11b?