hhuumm, thats a big question. I've never really used exchange so I can't answer that part, maybe someone else can, but as for the rest I can help you out with.
You could setup Samba & OpenLDAP as a Primary Domain controller, there is heaps of documentation out there for that, I'll post a few links to my website below that should help you out. One of the major disadvantages at the moment is there is no Windows 2k/2k3 Group Polices when using a samba PDC, but there are other ways of going out about, that I will also post below. advantages are the obvious ones.
* Easily integrate Linux hosts into your LAN.
* Save you from expensive, Byzantine Microsoft licensing and fear of the License Police.
* Greater stability, reliability, and performance.
* Multiple choices of database backends: tdbsam, ldapsam, or mysqlsam.
* Community and commercial support.
* Secure remote administration via SSH.
* Distributed authentication: running multiple secondary Samba servers is much simpler to manage than NT4 primary and backup domain controllers. You don't have to hassle with silliness like promotions and re-installations – just tweak a configuration file.
* Reliable, efficient synchronization of Samba servers via rsync.
* If you start out with Samba as your domain controller, then decide you want to upgrade to Active Directory, no problem — Samba 3 snugs right into Active Directory with a few configuration tweaks. Unlike NT4, which requires a complete re-installation to change its role from a domain controller to a domain member.
I got that list from
I hope i've helped, i'll leave some links to some information on my website below, hope that will help in the setup
OpenLDAP LAM Samba as PDC
and if you feeling a little bit more adventurous later on you could try getting into the policies with samba
Samba Primary Domain Controller with Group Policies
if your still not sure about using exchange, you could try out other alternatives in Linux & there is heaps of information on my website about setting up Linux mail servers
This one you might find the most useful
but there is more information here.