Originally posted by Peter Zack 300mm IMO is the starting point for a long tele lens. To get nice up close wildlife shots, 300mm might work for some of it but certainly not all. Particularly some of the wildlife you would see there and not want to be very close to.
As you say, this is a once in a lifetime chance. The question you need to ask yourself is. Do I want the 50-500mm after the trip? Will I use it later? Or is the $125.00 as a rental a better purchase? Finally most new lenses hold their value very well if you get a decent deal on a purchase. But it is a gamble if you buy and resell later. You could easily 'spend' more than $125.00
Finally the 50-500mm is a very good lens and has a great range where you wouldn't need to change lenses often, but to add to the weight is the need for some type of support. A monopod or tripod would be advisable.
What he said...
Regarding camera/lens support. The bird photogs in my neck of the woods use a beanbag device to cradle the lens on the window frame or window of the car.
I have some friends that were in Haines last winter for eagle photography. They rented some big Canon L glass (~500 at the long end) for the occasion. I went to their slide show and they made a point of saying that while the big lens was nice to have, it was a pain to lug around. As far as support, they also borrowed a gimbal head.
While in Haines, they also did some other wildlife shots of goats and lynx and such...at the local game ranch. Game ranches are the "dirty little secret" of wildlife photography and film making. Remember Marty Stouffer (Wild America) on TV? Most of his animal sequences were made within a fenced reserve with captive animals. (You can see the fences if you look closely at the background.) While it might not seem quite right, you could spend a lifetime attempting to get a full face shot of a wolf in the wild and might not live to tell of the experience.
Steve