CentOS 5 :: Centralized Logins And Group Management?

I have to set up a box which can manage all the logins in our company and has the feature to manage every possible permission with as much comfort as possible. We are using Linux and AIX therefore my Boss is willing to switch from our Windows DC to a Linux DC. And here lies the problem, I don't really know what is needed to set the Box up to manage the Unix, Samba and LDAP accounts with one tool maybe?

I would like to know which Software exactly is needed and how to manage to get the thing to work together with a security aspect. I configured a Samba DC with LDAP, Kerberos and TLS but it looks like I overdid it because Kerberos is not able to manage the things we need in a manner that the other Admins in my Company would get things done in a short time.

Therefore I would like to get listed all the Software needed and maybe some How Tos how to get thing working, because I am losing my nerves on this matter.
In the last 3 weeks I have set up several test boxes but every time something doesn't work. My biggest Problem is to get Samba and LDAP to work together with TLS or another security scenario.