Hi Penelope,
I haven't done this tour yet but it's definitely on my wish list! If you are concerned about altitude sickness, talk to your doctor about Diamox which can be prescribed to prevent and alleviate the symptoms. I know a lot of people who have done this tour and they have loved it - only a small number get altitude sickness and I do know that oxygen masks are readily available, as is medical assistance if needed.
Best wishes,
Bob