hmm, it depends on the location of each machine. If they are all in the same room, a wireless router would probably be the cheapest and quickest way to go, if they are in seperate rooms you will probably have to go wired, which will cost a lot more. I would assume £500 + to install the necessary wiring. You could set up one as a server or just go peer to peer (which is probably the easiest way).
If you have free space on your legacy network you could try connecting in to that instead and running a Linux server as the Linux server will quite happily deal with Apple network traffic (even a windows based system will - but if you are using RM network legacy system I'm not sure if it will work or not).
Regardless, you are going to need some sort of router (wired, wireless, or legacy) and some method of connecting the machines ( wired or wi-fi). The actual software set-up Im not so sure of, but knowing Apple, its probably 20 times more easy to do than a M$ system.
I hope this helps.