We took a walk on the Susquehanna River at the Hiawatha Landing with the 100-400mm MII only. There are many questions about this lens and misunderstandings. I am showing the versatility if you have just this lens on a walk.
First is at 100mm of a Cherry Tree.
Second is a Bumble Bee.
Third is a Dandelion
Fourth is a Goose way out on the river at 400mm.
Fifth is a Trout Lilly.
The flowers are quite small, perhaps about an inch or a bit more across, the bumble bee most know how small they are, the goose was about 1/2 way across the river.
This should help answer questions about what a great lens this is for a walk on the river or in the woods. The close-ups were all at about 400mm.

I posted on this topic earlier this year. So....re-post.

Dustin Abbott's youtube piece offers a very valuable assessment on this very topic. After reviewing it, a friend of mine and I went into the field to test it for ourselves. He currently owns the Canon 100-400 L II, paid about $1800 for it, and loves it. After using it for a fews hours (mounted on Canon D5 MIII), it was clear to me that is a terrific lens! 9.9 on a scale of 10. I shoot Nikon (so do not have that choice) and decided to purchase the Sigma lens for B&H (Nikon version) with the dock, B&W filter, 4 year extended warranty, and cleaning kit for $670 plus change. At the end of the field test, we both concluded the Sigma results were quite close to the Canon it terms of overall performance. IQ was almost identical in nearly everything did. But the AF came up short on the Sigma. More than I liked. But then again the Canon's was utterly amazing. Never missed. Sigma's build quality is not what one would call "PRO" like the Canon. But that also lends it to being extremely light and manageable for field use. The Sigma did surprisingly well in close ups, better than specs would indicate. Bokeh was very pleasing. I have read/heard about quite a bit of Chromatic aberration, but we saw little evidence of anything excessive. Easily corrected in post. The Sigma is a REAL value. AND mine worked great right out of the box on D800e and D4 bodies. No dock adjustment needed. I am keeping the lens and using it a lot.

Thank you for all the advice given in response to my request.
It looks like this lens is what I'm looking for:

100-400MM F/4.5-5.6 L IS USM EF MOUNT LENS {77}

KEH has used it for under $900
Canon has it new for over $20000

Do many of you go for used gear? I only started to do (almost all) manual shooting about a year ago so I'm new at it. I've never shopped for a lens let alone a used one. On the other hand I'm an amateur and sending over $2000 is a bit daunting.
Please advise.

Absolutely.
I use it all the time for birds, wildlife, scenery, close-up of wild flowers and spiders.
Well built like a tank (See Lens Rentals Review) and sharp as a tack. There is no other lens with the versatility of it by any maker especially the amazing close focus ability which is closer than the 70-200 you are looking at and it does it at 400mm so the magnification is much greater.
It sits on my camera a lot of the time, in fact it is attached right now on the coffee table ready for the critters around the house in the spring.