I've been around. There was a bit of a dry spell and people were also emailing directly so I just answered there instead.
So my first recommendation would be to do this in stages. Learn how Server 2012 Essentials works before jumping into Hyper V. Hyper V is a good choice for a HyperVisor since there are several free and affordable backup solutions. Windows Server Backup is great and BackupAssist is an excellent commercial product. Lots of people jump on ESXi or Citrix without thinking about how to back it up. Replication is also a premium feature in most cases.
Specifically is this architecture ok? Server running hyper-v then 2012 essentials with AD and AC virtualized.
Does one need absolutely need 2 VMs to separate AD and AC roles? I'm thinking of running 1 VM to keep it simple.
There's no need to separate the roles. There's a few of us who run SBS 2011 Standard (Exchange, SharePoint, AC, AD, DNS, DHCP, etc.) all on the same server. The important thing is to make sure the server has sufficient resources (RAM, CPU, HDDs, etc.). However, I am currently keeping AC off Server 2012 Essentials as Version 6.3.3 (SQL 2005) is officially not compatible with Server 2012.
What you can do is get Server 2012 Standard and use your downgrade rights to get Server 2012 Essentials and Server 2008R2 Enterprise in two instances. This would also give you access to the full GUI of Server 2012 on your Hyper V host.
Server 2012 Standard HV Host
-> Server 2012 Essentials (AD, DNS, File Server, Backup, etc.)
-> Server 2008R2 (AC and SQL 2005)
The alternative would be using free Hyper V Server. Hyper V Server has only Command Line, no GUI. So it's recommended for advanced users. Even though the majority of the setup can be completed from Hyper V Manager in Windows 8.
Hyper V Server 2012
-> Server 2012 Essentials (AD, DNS, File Server, Backup, etc.)
-> Windows 7 Professional (AC and SQL 2005)
I would recommend the first layout as it's easier to manage and only slightly more expensive than the second layout. Probably 100 dollars between Standard and (Essentials + Win 7 Combo). But it would be a lot easier to setup the first.