You probably need a cable/DSL router, in addition to a DSL modem. Here in the States, the ISP provides/discounts their preferred modem, which (as far as I've seen) has only ONE ethernet port. I got a Linksys 4-port router, which handles the authentication for my DSL (PPPoE is pretty standard here) as well as sharing everything.
As I understand it, in Australia and the UK, ISPs seem to like hanging the cost of the modem on the customer, which leaves the customer wondering what to buy, and whether he's being gouged. Sorry about that. There have been a lot of posts here about what modem is best for British Telecom DSL, and several about Australian ISPs as well. If the search function doesn't help you out (which is understandable), don't worry. You'll get three or four hits pretty quickly with plenty of advice on what to get.
You DO NOT have to worry about platforms. Ethernet networking is very well standardized, and all the computers you listed are well behaved and play well together. I have two PC desktops wired to the router, plus a wireless access point connecting an iBook and a PC laptop wirelessly. No sweat, no fuss, and not too much work to set up. If you need help with settings, we're here for you.