EPDM is better at handling large vaults than Workgroup. If you have a server that can handle two Workgroup vaults of this size, likely it should be fine handling one EPDM vault. Your first goal is certainly achievable.
PDMWIMPORT imports files into Workgroup, not EPDM.
The "Update References" tool can be used to fix/change references after the files are in EPDM.
I am going through this right now, There are several tools that we are using to get us there. Our end goal is your "Ideal Solution", and we are currently tracking to have this deployed in the next month. If you would like additional information on the software that we are using please fell free to send me a note.