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Wonderful GSD. I come from a home that seems to be a historic haunt of theirs, having been owned by at least one of each variant over the last 35 years.
Re your lens choice.....If you are only going with one lens and want to use a prime, then the 85mm 1.8 is really an ideal way to go. It is about the perfect focal length for most canine work in the home and studio.
IF.....you can stretch a bit, then look at the 24-120. It will give you a bit more composition flexibility and a bit more "reach." Optically, I don't find any fault with it till I start trying to enlarge above 24x36.
Lastly, a few kind notes on the photo above. Nice head angle, great ears, good eyes. Really icky background (you could try for more distance between dog and wall, etc. to soften background if you can't control posing position). In my dog show work when photographing large dogs I normally try to work at f/8 or so in order to let me focus on the muzzle just in front of the eyes in order to keep the eyes in really sharp focus, the nose in acceptable focus, and the ears in acceptable focus. I find that it is really distracting to see a gorgeous portrait of a dog blown up to 16x24 or larger, with wonderfully sharp eyes --- and a blurred nose pointed right at you. Also, on this shot I would have cropped in from the sides to make a vertical composition.