The way Richard explained it to me (and Jim at Tiger Racing said the same), the one major component can be the "drivetrain' and in that they include the gearbox, diff and halfshafts. If you then put in a new Engine and have all the other parts new, you should qualify for a current registration, which is what GBS did with the current 60 plated demonstrator.
As for donor cars, GBS do have a large stock of old Sierras and a donor pack is £500. They can even get it all shot blasted and cleaned up for you (for a small price) although it is not "refurbished" as such, but it looks like it is.