How can I setup different "environments"
There is an application Dockspaces that allows you to switch between different sets of applications in your dock (and I think different desktops but I don't use the desktop at all so I don't know if this does what you want)
In "Computer -> Users" there's a folder called "Shared Folders". You can put shared files for all users in there (including the iTunes library etc.). With fast user switching enabled it's very easy to handle multiple environments.