Building an OpenSource Network (Intranet) infrastructure based on Linux (AD, Domains, etc.)

We currently manage our networks (Intranet) using Windows Server 2008 R2, as it was relatively easy to manage and configure. Being a non-profit organization we were sponsored our initial setup but we would have to add more computers soon and we would like to migrate most of our hardware to Open Source servers and clients.

We currently use a Domain Controller and a File Server to create and manage user accounts and permissions (using GPC on groups) and manage files on the network. We have multiple users using the same computer so this suits us perfectly. (Mapped network drives are such a blessing!)

We are looking at Ubuntu as the distro for the client computers, but we would like suggestions of what open source server distribution I should use?

The requirements would be:

1) An easy to setup and use Domain Controller, I know it won't be as easy to use as Windows Server, but the smaller the learning curve the faster I can setup the network.

2) Something similar to User Groups and GPC's which would automate the permissions to be set on the network.

3) I was looking at SAMBA for a File Server, but prefer a pre-built and customized distro to handle the same.

Actually I need something that is pre-built and has the least amount of customizations to be made to it and the least amount of setup time. It should also be some what similar to Windows (GUI based) (although that would be too much to ask for I think!).

Please also post links to user friendly documentation (technical jargon is a little tiresome to work with) and how-to's to speed up the setup process. I am willing to put in effort for the setup process but the easier it is initially the faster I can complete the initial setup and migrate the existing network.

I have been managing this setup for about 6 months and so am familiar with a lot of networking concepts, but UNIX/LINUX is entirely new to me and would like your suggestions about this.

Edit: One very useful feature would also be if I can have a log of user login's and logout's, applications used and browsing history.