Well, I think I'll weigh in here also. To look for used equipment from a reliable source I always check KEH.com in Atlanta (they have been in business for something like 30 yrs and give a 6 month warranty) or Roberts Imaging in Indianapolis for both new and used. Always call the folks when looking for used as the inventory changes daily. As to lens choice, Canon's 70-200 family consists of 4 (actually 5) lenses, all optically very nice. These lenses are the 70-200 F:4 and F:4
IS, and the 70-200 F:2.8 and F:2.8
IS MkII (the 5th lens is the original F:2.8
IS).
IF, and only IF you only plan to use the lens under nearly ideal lighting conditions, there is very little reason to spend the extra money to get one of the IS versions or the faster (F:2.8 IS) members of the family. I bet that you will eventually want to use the lens when the light is somewhat dim (just before dawn, just at dusk, for instance) so, I would strongly recommend that you look either for the original 70-200 F:2.8 IS (which will be used) or a used F:2.8 IS MkII (I just picked up a very nice used MkII from Robert's Imaging for a bit under $1,700.00). Now, I know that sounds like a lot of scratch (or at least it does to me) but good glass is an investment, and a good, pro built lens will last you for many years and actually hold it's value (especially if purchased used). If you just can't justify one of the IS F:2.8's, at least try to find a used
IS F:4. You should be aware that Image Stabilization only counters blur caused by camera shake, not subject movement, but in telephoto lenses, camera shake is a major cause of image degradation. To counter motion blur caused by subject movement, you need as high a shutterspeed as the light allows and THAT is where the "faster" (lower F:#) lenses come in. An F:2.8 lens allows the passage of twice the light as an F:4 lens, thus allowing twice the shutter speed. Should you purchase any of the 70-200 L series lenses, it should last you through this camera body and at least the next one, which is why I suggest getting the very best one you can possibly afford, even if you have to wait a bit. Watch the used departments. Deals change all the time.
I concur with those advising you to GET OFF P or the "green box" and learn to use either Tv (you set the shutter speed and the camera chooses the appropriate aperture) or Av (you choose the aperture and the camera chooses the appropriate shutterspeed). Personally, I almost always use Av as I generally want the highest possible shutterspeed for the light conditions, so I set the aperture at wide open (assuring the max amount of light gets through the lens) and allow the shutterspeed to "float".
As far as software, I strongly recommend Adobe Lightroom (I would also recommend using the creative cloud version). FIRST, (and this is VERY important) before you even sign up for Lightroom check out Tim Grey's introduction to lightroom on You Tube. It's about 2hrs long, but WATCH THE WHOLE THING! It will pay HUGE dividends. Here is the link:
https://www.youtube.com/watch?v=QSwkDC3q7uk
Hope this helped some.