First of all, this is a terrible idea. If you don't know how to manage Linux and you don't understand the implications, you shouldn't be doing this. An operating system is only as secure as someone that knows how to manage it. If you're familiar with Windows and unfamiliar with Linux, then the odds are that you'll be much worse-off with Linux while you learn.
Also, keep in mind that configuration management is handled much differently. You'll need to learn something like Puppet, Chef, or radmind instead of using tools like Group Policy or SCCM.
Basically, you'll need an authorization database to replace AD. You can use any number of LDAP servers out there. Go do some research.
You'll need productivity applications. Does OpenOffice meet all of your requirements?
You'll need to train your employees. Are you OK with the workflow interruption that is going to happen as you uproot their entire technology infrastructure?
Are you going to learn things like Zimbra/Dovecot/Postfix/Sendmail instead of Exchange? Are you going to learn to manage Apache instead of IIS?
If you have a compelling business case, then sure, go for it. But do your research. There's nothing inherently more secure or better about Windows or Linux. It's only as good as the person managing it and the quality of that management will drop sharply as you learn a whole new suite of tools.