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Consider renting if your goal is the best gear and if your main objective is to sell and make a profit. Twelve hundred dollars is not much of a budget and as cool as a trip to photograph Gorillas may be, you will not be the first to bring such images to the market and being your first trip working with Gorillas , what do you expect to accomplish ? Do you have a few weeks a month a year ?
In general you want bigger lenses , primes or zooms in the 300 - 500 range maybe even 600 and super fast. That will cost you a fortune to buy. Plus you need to practice with such gear before your trip and review as many Gorilla shots that you can before you go, so you can sharpen your eye as to what kinds of images are in demand and make great shots. Then your shots have to stand out against thousands of shots by other photographers , some that spend their entire life taking gorilla shots AND you have to sell your images at a high enough price to cover the cost of the entire trip and make a profit.
May be best you buy the best lens you can within your budget and relax, have fun , get the best images you can and enjoy your vacation. Something like a 300 mm f4 might fit your budget or one of the zooms that fit in that range that are similar in speed. If you must take pictures from a safari vehicle a zoom is really advantageous. You really need to inquire what the shooting conditions will be like, they might get you close enough that some smaller glass would be good to have with you. Get as much info as you can from the tour people before you decide on what gear to take.
Most of all have fun , life is too short not to enjoy your adventure.
You could use a satelite. I dont remember the math needed to compute the necessary mirror diameter that one can get good resolution on the ground, but I would estimate 100m should get you pretty close to sufficient resolution. Thanks to getting the perspective from above your images would be unusal and unique, too !
You can then also lend this satelite to scientists whenever you dont feel like photographing gorillas youself. I bet they'd love to get their hands on a 100mm optical telescope located in space. Just not be surprised if they dont actually want to point that thing at earth.