MiamiDx is only needed if you want to use it as a router or firewall. Today an ADSL or broadband router is much cheaper than MiamiDx was at any time. So you should look at this first. Miami or Genesis (as contained in AmigaOS 3.9) are absolutely sufficient for a simple network connection.
@odin: a second-hand key of MiamiDx might be a better solution, although you have to live with the previous owner's name in the key anyways.
Bye,
Thomas