How many servers to install?

Dear community,

we are planning a test lab Installation of Ex2013SP1.
2012R2 AD
70 Windows 7 Clients
OWA from Intranet and Internet through DMZ
about 600GB Mailbox data

Now wondering how many Server we would need.
Of course at least two, Edge Transport and CAS+MBX.

But does it make sense to separate CAS and Mailboxes? Place the CAS into the DMZ because of Internet Access to OWA?
Should CAS be on a Member Server or, like Edge, a Workgoup Server without AD?

Please help

Thanks in advance
Davorin