Choosing my first Wildlife Photography Lens

Started 1 month ago | Discussions thread
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kkv13 New Member • Posts: 6
Choosing my first Wildlife Photography Lens

I am a professional wedding photographer from India.
My love for Photography started with Wildlife and Nature, and I was really wanting to do these professionally. But after understanding that trying to make a living from these branches of photography was not at all easy here in my country, and requires a very large investment, I decided to start doing some commercial photography and weddings, from which I have managed to save up some cash to finally invest in my real passion.
My choice is primarily between:
A) a New 200-500mm F5.6
B) a Used 200-400 F4 VRII
I am getting the Used 200-400 at a little over TWICE what a new 200-500 is costing me.
( getting 200-400 at approx $2800 while 200-500 is closer to $1200)
I have been racking over this for a couple of days and going over forums and videos, and while there are quite a few posts even here, everyone has different requirements and nothing is quite clarifying it for me.
I know it's my choice at the end, but any help from more experienced shooters would be much appreciated.
Just to sum up:
- I use a D850
- I cannot rent these and try as they are not available in my country for rent
- my ONLY concern is photographing wildlife and more mammals than birds
- weight is not an issue
- yes, I CAN afford the 200-400, but it would clean me out pretty much. My only concern is that even though I'm getting it used and much cheaper than the new one, IS IT WORTH the difference? 
I just don't want to end up spending twice the amount of money and getting barely some difference.
My instinct is telling me a pro F4 would be worth it and i can affort to lose the extra 100mm since birds are second priority.
Would be really grateful for any suggestions, and I apologize for the long post.
Thank you.

 kkv13's gear list:kkv13's gear list
Nikon D750 Nikon D850 Nikon AF-S Nikkor 24-120mm F4G ED VR Nikon AF-S Nikkor 70-200mm F4G ED VR Tamron 15-30mm F2.8 +2 more
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