You will need a power supply/base station, headsets, and boxes, and XLR cable to interconnect everything. I suggest production intercom, they make great gear and you can pick it up much cheaper then telex or clear-com.
This is a link to production intercoms "econo-com" line, this is still bulletproof gear. http://www.beltpack.com/econo.htm
I was really impressed with the products from Eartec at LDI. Unfortunately, I haven't found anyone who owns the products yet to give me a first hand account of how they work in the real world. But they have a 4 person wireless full duplex system for $800 (including belt packs and headsets no base station needed). You can't beat that. For About $550 you can add an interface that allows the wireless system to talk IN FULL DUPLEX with a wired system.