Hello, I have the following issue: I would like to upgrade my camera, which now is a 7D, and the likely candidates would be 6D, 7D mk II, or an used 5D mk IV or 1 DX, from a shop in Vienna, Photoborse.at. My main lens is the Canon 100-400 IS mk. II.
My primary interest is not necesarily BIF, I like more large mamals, roe deer, rabbits (that is when I can get them in focus). About the 7D mk II I have an aprehension after reading I believe all I could find about it on the forum and Arash's and other photographers reviews; I'm afraid that I might get a bad copy ( after all, Doug Brown went through 3 bodies till he found a good one). The 6 D has good image quality, even at high ISO and good AF, but a really low framerate. About the 5D mk IV and the 1 DX I've read Arash's reviews but given the fact that I'm an amateur enthusiast on a budget, I'm not sure about the 1 DX, could be an overkill for me. My main concern about FF is that, given my lens, the subject will appear smaller in the frame than with the 7D, and it will require me to get closer, which sometimes is hard or impossible (roe deer and such tend to be skitish), so the advantage of better image quality, even at high ISO of the FF camera will be lost. I was thinking about using the 5Dmk IV/1DX with the 1-4 mk II and a 1.4 TC II, for some extra reach ( Arthur Morris has some outstanding images with this set up), can you tell me if its a valid combo as far as AF speed, accuracy and image quality goes? Would a 1.4 TC III make things better, or there is no difference in image quality between the II and the III with an older camera like the 7D? Another option would be to keep the 7 D for now and invest into a used super telephoto lens. I found 2 stores in Viena, Photoborse.at and unitedcamera.at, which have a used 500 f4 L IS, for about 4590/4700 euros, the product information page states that they are in good condition. Perhaps somebody might be from Austria and has had dealings with these shops? An advice as to what to look for when checking the lens (apart from the front and back for dust, fungus and scratches) would be welcome. I find this option quite atractive, a 500 at f4 would give more reach than 400 at f 5.6 (though I read a post on BAA which states that the actual focal lenght of the 1-4 mk II is about 380-390 mm, not 400). Given the vast experience of the members of this forum I would like an advice as to what to choose for a new camera, more exactly which one (though question) or a used super telephoto lens?

Thank you
Bretoteanu Dan