You can use a wireless access point to set up a wireless network, you don’t need a router to set up a small network. The best thing you can use is a lynksys access point. If you specify the distance of the imac sutiable wireless range extender can be suggested.
The advantage of using a combination access point/router is that you can setup different subnets on either side of the router and perform some enhanced security filtering. Just another means of providing layered defenses.