Samba should be able to provide most of what's on your list but I'm not sure about the "management of users" part. I'm just not familiar enough with current Samba releases to know what functions it performs, in this regard.
I suggest reading the Samba doc, namely
this guide. It talks about configuring Samba in various scenarios and maybe one of those scenarios fits your needs. If it does, you can get an idea of what would be involved. Samba can serve as a PDC or a BDC and can join an AD.
If you're struggling with Windows 2003 server and AD, you might want to spend your resources learning more about AD (maybe take a class). That might help more than switching server platforms.
Good luck!
Peace...