Exchange server 2013 architecture planning

Hi all,

We are planning for a new Exchange 2013 installation :

• 1 site
• around 2000 users
• 1 domain

I need some hints about the optimal architecture keeping in mind high availablity and future extensions.  

examples :

1. 4 servers with CAS and MAILBOX roles
2. 2 CAS only servers + 4 mailbox only servers

Many thx in advance