[X4U] connecting older iMac to wireless network

[Kansas Territory]

have a friend who is trying to connect his older iMac running some  
version of OSX , to a wireless network of some kind which is  
available in his office building.

Being an older iMac, doesn't have built in wireless etc..

what are his options ?

I thought maybe the Airport Express Base Station ?   all I could find  
on apple.com store is the Airport Express Base Station with AirTunes  
for $100

He's not going to be connecting to another Apple wirless network,  
this is some wireless network available to tenants in the building.   
needs to connect via DHCP.

any suggestions for a solution for this older iMac ?

Kansast