First,
A warm welcome to OPF! We're glad to have you on board. There's an entire year to go before your wonderful trip, so you have time to test out your choices.
I know little about Nikon lenses except for the legendary 14-24mm f2.8 G series Zoom which is wonderful for landscapes, the D700 which is revolutionary in performance and price and the D800 in resolution. These modern cameras have very competent focussing and that's an absolute necessity in photographing on a safari.
You might consider renting lenses and bodies from
BorrowLenses.com as their quality is superb, service reliable and prices reasonable.
If your trip is 10 days or less, the price of almost any lens, (except like a new 800mm 5.6 Nikon) is a bout $35 to $135 for 10 days and they have lots of local partner companies you might be close enough to pick up your order nearby.
I'd try to rent these lenses and visit a local zoo or wild animal rescue reserve to be able to get a feel for how you like the lenses and camera combinations. I'd personally want at least a D700, but I do not know Nikon cameras well. Renting a better body is also worthwhile. You airfare or boat trip to the Galopagos is going to be far more than the price to rent really nice gear that you are most comfortable.
I'd think you want zoom with a focal length of up to 400 mm, at least, for game and then a 14-24 for landscape and game migrating or crossing a river. Two cameras, two lenses.
An APS C compact camera like a NEX or Ricoh GR would be great for in town causal shooting.
Asher