Hi. Welcome to 2010. DO NOT BUY A PHONE SYSTEM.
Get with a VOIP provider, get IP phones, and your KSU lives in the cloud. It will be the last investment you'll need to make. It's reconfigurable forever, and you can run a SIP phone on your laptop, or a SIP app on your smart phone and make and receive calls as if you were in your office.
When you get with a business VOIP provider, since all the "features" are software configurable on their end, if you can think it, they can make your (virtual) system do it. All the requirements you listed are dirt-simple to make happen on a business VOIP system.
My VOIP service comes from these guys: http://www.usgrp.net/
Oh... the greatest thing about a virtual PBX over VOIP... it's cheap as hell.