I think as an existing iOS user, you would find the alternatives frustrating as none have the same fit and polish, and of course, none of your existing investment in apps would carry over. Some folks like diversity and playing with varying operating systems at the same time. I prefer consistency; makes me faster.

Outlook without Exchange is a bit of a dead end. Well, wirelessly at least. You can always sync your iPhone or iPad to it, but that requires a wired sync via iTunes, which many find inconvenient. Using Google Calendar and a different desktop app would allow you to have the calendar on all devices syncing wireless and automatically.

At the moment, the only two viable options are iOS and Android, and I'm just not familiar with the Android offering in Australia. With Android, unless you get a Nexus One or a Nexus S, you're going to be beholden to your carrier for operating system updates, and in such a fast moving industry, that can be frustrating. If you do go Android, research a handset that runs the latest OS, and is expected to get free updates in a timely manner. The remaining options are RIM, Microsoft and Palm/HP, although I'm unsure if the latter have an Australian offering. Microsoft and RIM are 3 years behind everyone else, and while Palm OS is pretty slick, the app market is not comparable to Apple or Google's at all.

Sorry, this is a bit of a rambling mishmash of insight. Hope some of it is of value to you.