1. The installation mode that lets you install VSA in an existing VM environment is brownfield installation. Rawlinson from VMware Product Management has done two posts on the same over here.
2. Externally connected storage should not be affected with VSA implementation.
3. vSphere Storage Appliance 5.1 lets you expand the storage of existing appliance. There might be some downtime of VMs involved, however it is supported.
Most important thing to consider in two node VSA is that the cluster service needs to run outside of your VSA hosts. So, you need to have a small desktop kind of box or any other server to run your cluster service. Things are well documented in the Technical Deep Dive paper as well by Cormac here ...
http://www.vmware.com/files/pdf/techpaper/VM-vSphere-Storage-Appliance-Deep-Dive-WP.pdf
DA