If you can get a DSL or cable account that’s about all you’ll need for network access to start with. A high end home router can be used for the router and firewall. There are higher end devices that you can look at which will have more options.
If you want a Windows domain look at Windows Small Business Server which will provide you with Exchange, a Windows Domain for Authentication, a SQL Server, etc.
You’ll then just need enough network switches so that you can plug everything in. A couple of 48 port network switches should do the trick.