How far can one expect a external examiner to travel to be on defence/viva committee?
If your university is paying the expenses, then it's quite plausible for an examiner to come from arbitrarily far away.  I myself have gone from the US to Europe to serve as an external examiner, and it was no more a burden than traveling to a foreign conference.  You can also combine other things into the trip: for example, when I served, my colleague also asked me to give an invited seminar, and we took the opportunity to work on some collaborations as well.