Mac OS X Server can provide many of those services for you. Many folks use Mac OS X Server's Mail service as a great filtering, virus checking and spam blocking solution in front of another Mail server (like Exchange). It can be used on its own, too.
Leopard ships with Apache 1 and 2, with 2 running as the default. It also has a built in FTP service. There are no additional fees for any of these services or any service built into OS X Server.
Leopard is able to be bound into AD as a full domain member. You can leverage the user accounts for authentication and perform a full integration of OS X into the AD Kerberos Realm. It cannot be a backup AD server.
The Server OS can be put on many different types of hardware. Your best bet is to get an Xserve. That will include the hardware and the full Unlimited OS license in the price. It's a great 1U server with 8-core processing and server grade parts.
You can read more about Leopard Server at:
http://www.apple.com/server/macosx/
You can read up on Xserve hardware at:
http://www.apple.com/xserve/
If you are unfamiliar with the system, which it sounds like you are, you should get in contact with Apple and get an account team to work with. They can arrange resources like briefings on using the OS and possibly try and buy on the hardware to be sure it meets your needs.
Let me know if you have any further questions. I work with OS X Server all day.
Michael