The main differences between Standard and Enterprise is that Standard can have only 1 storage group, 1 mailbox store and 1 Public store and is limited to 16GB(75GB With sp2) and it doesn't support clustering of servers.
Enterprise supports 4 storage groups, 5 database stores and 16TB of storage and supports clustering. It is also a lot more expensive.
For 100 users standard will be more than sufficient.
It is relatively easy to set up, but it depends on each organisation. The most time is setting up users and mailboxes, and ensuring you are not relaying mail for spammers
It gives you more than Outlook on its own, as you can share calenders, contacts, send appointments, share email.
Cost wise, the standard server licence is about £750-800 and each CAL is about £75-80 each!
Plus if you haven't got any Windows 2003 server cal's you would need another server license (about £500) and CAL's for windows (about £35 each!)