I would like some advice on the feasability of the following migration scenario.

Current setup is based on an SBS 2003 Server (an old HP ML310) in a single office with a second Windows 2008 Server (a Dell PE2950 III) running a complex SQL Server 2008 in-house application (which supports the primary purpose for the existence of the business and which is constantly under revision to meet clients changing needs (it's a kind of data warehouse)).  This network is currently protected by a Watchguard Firebox Core X550e.  We want to implement a business continuity / disaster recovery plan that will enable the branch office to take over operations in the event that the key person in the primary office become ill or unavailable for an extended period (i.e., months rather days).  The branch office currently runs as a simple workgroup setup.  We propose to purchase new server hardware for both the main office (Dell R410) and the branch office (Dell T410).  The branch office will connect to the internet via a Watchguard Firebox Edge 20e.  The branch office will connect to the main office via VPN(s) managed by the Fireboxes.  My question(s) pertain to the licensing and setup of the branch office OS and SQL Server config.  After a bit of research the following seems like it might be a reasonable way to go.

Upgrade the primary office to SBS 2008 using the standard portion of an SBS 2008 Premium licence.  Install a Windows 2008 Server in the branch office, configured as a read-only domain controller within the main office's SBS 2008 domain.  SQL Server 2008 can't be installed on an RODC.  Install the premium component of the SBS 2008 under hyper-v on the branch office server.  The alternative is to purchase a second full copy of SQL Server 2008 to run under hyper-v at the branch office.  Whilst the difference in price is not great (perhaps $1000 - $1500) we are at the very limit of our budget and need to save where we can.

Your thoughts on the above will be greatly appreciated.