It is a must to think about redundancy, so if I understand you correctly, you plan a single DC in this environment? Hmmm... I would not dare going below at least 3 DC in your case.
I agree, you should have redundant DC's in any environment of this size. If you have multile locations, possible add a rdc to reduce traffic and provide service if a connection goes down between locations.
Here's an excellent AD sizing tool, which you can download and help you plan your config:
http://www.petri.co.il/active_directory_sizer_tool.htm