Transition from Windows-based network to Linux-based

I am looking to make the switch from a Windows based network to a Linux (preferably Ubuntu, because I used it the most) based one. Currently the network includes a Windows 2003 server which acts as a domain controller and file server, a network shared printer (Canon 2300N) and 7 clients running Windows. Several other Windows-based clients connect remotely via RDP.

I would like to setup a server to act as a domain controller, file server, and (if possible) terminal server. The clients should be authenticated via the server and have access to the files stored on it (via ssh?).

Here are several points that I was wondering about:

  • All the clients use Outlook, some of the clients run Quickbooks Pro and another program called Tentant File Pro. I was thinking about replacing Outlook with Thunderbird (I contact, calendar, and tasks) and I know that Wine can't run Quickbooks and Tenant File Pro so I may have to use VirtualBox/VMWare because these applications can't be replaced by native Linux versions.
  • I need an adequate backup solution for the files on the server running as a daily backup.
  • Should I use webmin to administer the server?
  • What antivirus / firewall solution should I use for the server and the clients?
  • How can I allow remote clients the login to the server to be able to run applications (Outlook, Browser, Tenant File Pro, etc.) remotely on the server?

What is the best solution for this? And what would be the least painful way of making the transition for the users?

I know this is a lot, but any feedback would be appreciated.

Thanks.

Linux has domain controllers?

From what I've gathered I will be able to use OpenLDAP and Samba to replace the Windows Domain Controller, but I need to know if this the right solution and what about the other issues that I have.