If you want to move fully into the cloud. Don't set up anything local other than a router, your cable modem and network cables. You will save a ton of money. the local server isn't worth the crashes and IT cost.
As for blocking confidential data from leaving your domain, Lock it down in your administration control panel. If you set it so files cannot be shared outside the domain and have some information theft happening, The problem doesn't have anything to do with your network.
One of the major benefits (and really the whole point) of the cloud is to make your company data accessible to your employees from anywhere. Having Google Apps allows your users to log in at home, on their tablets, on their phones etc. Blocking personal email in the workplace doesn't accomplish anything that they couldn't do at home or on their phone at the park.
If employees cannot be trusted to keep data inside your domain, then they cannot be trusted to keep their username and password to google apps secure either. That would be a bigger threat.
What is an ideal LAN infrastructure for my business when I use Google Apps?
