What is a "Lion German Shepherd"?
As far as buying a 2 year old, it can be a good thing or a bad thing. It depends on the individual dog and how he was raised and trained.
You will get the best chance to judge his nerve and temperament if you are able to meet and interact with him in a strange location, rather than on his own property where he will feel more comfortable than he would in a strange place.
Have his owner tell you how he was raised and socialized, show you his level of training and show you how to handle him and give him commands and how to praise and reward him properly based on his drives, personality and training. Also ask for proof of health testing and certifications, such as hip and elbow x-rays. One of the advantages to purchasing an older dog rather than a puppy is that they are mature enough to have a better idea on what their adult temperament, appearance and health will be so make sure that the health testing appropriate for his age has been done.
As for being "sad", all dogs need an adjustment period when they change homes. Each dog reacts to this stress differently. Some become more subdued, some become more active and may have trouble settling down. He shouldn't be aggressive unless he has an aggressive personality to begin with, which is something you would need to assess by visiting with the dog and talking with his owner. It WILL take him a while to bond to you, and you need to realize that and that for the first several weeks, perhaps even months, he will not be truly himself. He also won't respond to you as well as he does to the people who trained him and who he was previously bonded with until he forms a new bond with you and the two of you learn to work together.