I had great weather. You have to remember though, Alaska has 2 seasons: Winter and Preparing for winter.
If you are in a tour, your guides should be able to get you to the best viewing spots. That's what I did, and it was worth it.
The salmon were running when I was there, so all the animals come down to the rivers to eat.
You should be able to rent the 50-500 at the places mentioned above. It will be many times cheaper than buying one
You could take your 18-55 and 50-200 in lieu of the 18-135 I listed above, it should get you most of your landscapes and general walk-around stuff.
I will recommend a decent tripod and ball head though. those long lenses get HEAVY after a while.
Can't wait to see photos.
Bruce