Upload & Sell: On
I don't think anyone's brought it up, but consider the OLD 80-400mm "D" lens. If you use a tripod, and have the benefit of time when shooting it can be very nice. I had one and got respectable photographs. You can find these used for around $725-750
I've also had the 300mm f/4 - a super, tack sharp lens. I kept my 1.7tc on it and the thing was flawless, and delivered beautiful wildlife images. But, more money still.
Then I bought the new 80-400 "g" lens, and the same result. Fantastic images, plus the benefit of a zoom. Of course a LOT more money.
Now, throughout this all, I've had a 70-300mm vr Nikon lens (think I've had 5 copies) as a backup/light zoom. So, when it catches focus properly it's good/excellent, but I seem to miss a lot of shots also with it on both my D800e and my D7100.. It used to work extremely well on other bodies, like my D5100, D300s, D700. It's just not delivering consistently on these newer cameras. When I had my D600 I don't think I ever tried it so I can't comment there.
So, just know what you're pairing with the D600. If you get another chance at the bison, or more likely a different opportunity in Alaska, you want to be ready. You won't be thinking of money then. But, I can relate because you have to stay within a budget. However, with lenses, often times you do get what you pay for. Especially with wildlife.
One final thing. I know money is tight for you, but consider this. Buying a quality used/pre-owned lens for a good price means you can sell it later for almost the same that you paid. So, it's like a rental for the time you are in Alaska. Take good care of it and sell when your done.