hi wowzers
this is a great question--I never even thought of this when I started my phd!!!
---while it is a good idea to have some idea of your external at the beginning of the phd, it is also worthwhile remembering that your research will probably go through some development and EVOLVE (you might achieve a breakthrough--and make some one-of-a-kind-state-of-the-art discovery!!!) and then your thought-of-external-at-the-start may not be relevant at that point :-)
I did not pick my examiners, I left it to my supervisor. While the viva is important, I also trust that she will not pick someone who will fail me :-p :-)
love satchi