You should start your project by reading Best Practices for Virtualizing and Managing Exchange 2013. It's a little dense and abstract.
- Space consumption is a constant battle. To sway the odds in your favor we recommend:
- DNS vs Firewall. You want to get mail to flow locally, but not leave the testing sandbox. This is always tricky and you should make sure you've got it right before you go to any appreciable scale with your testing.
- Keep your Active Directory on a separate Virtual Machine from your Exchange components. Why?
- The Exchange VM already has enough to do – don’t add another function to its workload
- Next, a separate VM for AD should mimic your production architecture. It allows different administrators to manage discrete roles. You might as well start by following best practices.
Further good reading: