> 1. I currently have six virtual domains around 300 users all in all, will Zimbra Open Source with Win2k3 AD lookup work?
Yes do an external authentication
2. Can the Open Source Edition support such setup?
Would recommand to go for Network as you get other benefits too
3. The reason Win2k3 AD will be used is because we need to sync our AD to a remote AD, however if it's not supported and I omit the Win2K3 lookup, will Zimbra Open Source be able to sync its GAL with a remote AD?
Why dont you do an external GAL lookup rather than sync ?
4. Will outlook and entourage mail clients be able to connect to my desired setup and be able to use GAL?
Outlook cannot as with opensource you dont get zimbra connector. Thats why would advice to go with network edition.