Ralph, you are correct in your research. Niki Lauda never drove for Williams (who was the team sponsored by Saudia). I presume you are more interested in building that car than the Lauda cars (Ferrari's in the 70's and McLarens in his 80's return to F1)
Coinsidentally, I ordered that kit from F1 Specialties just a few weeks ago. So that is one option. (I had a very pleasant experience with them, btw) AMD also has resin kits of FW07's and FW08's. Much more expensive, though.
From (my own ageing) memory, there are more than one decal option on Tamiya's FW07. Ah, yes, here's the decal sheet from the above-mentioned page:
http://www.f1specialties.com/main/decals/014.php
If you something to ask about that kit, shoot then I can have a quick look tonight.