I completely understand your reluctance to drop $7k (hood included) on a lens and I would recommend that you be making some money with your photography to justify the cost. One thing to remember is the resale value of this type lens, it’ very good and I could easily sell it for 10-20% less than what I paid for it.
My next shorter lens is a Sigma 120-300 f/2.8 EX that does a phenomenal job and I use it often. It coupled with a 1.4X Tele Converter gives you 420mm at f/4.0. It’s a viable alternative to the more expensive fast super-telephoto primes. The Sigma will cost you around $2,000 new.
Another option is renting lenses. Look at your local photography shops and see if they rent, if not I bet they can tell you the nearest place that does.
It’s hard for me to say what the 600mm would do with your picture. If I understand you correctly and you shot that with a 3X zoom and it’s a 100% crop, the 600mm would most likely be too much magnification to get the composition right. In other words, you may just get a full frame shot of the sheep without the barn and flowers in the foreground.
I regularly ask landowners for permission to go onto their property and I can only think of one instance I was refused permission. Once they know who you are (give them a business card), that you are there only to take pictures and that you are responsible enough to not climb fences or leave gates open, then they will welcome you. I also try to return with prints and present to them as a show of my appreciation.
One final note, the 600mm lens will change your photography. Equipment does not make you a photographer, but without the equipment some shots are impossible. I have zero regrets for dropping $7k on this lens and would do it again if I needed to.
Hope this helps you.
Peace,
Ed
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The road to happiness lies in two simple principles: find what it is that interests you and that you can do well, and when you find it, put your whole soul into it -- every bit of energy and ambition and natural ability you have.