cer2225, Polycom looks like a good choice if I wanted to go outside of the MiNet scheme to SIP, but I'm not that brave yet since this will be my first large roll out of Mitel phone system.
IrwinMFlethcher, you made an excellent point that I did confirm. The 5304's do not offer full-duplex handsfree, so it's the 5312's for me.
Thanks for the comments.