A GSD would be my last choice as it is also the last choice for a seeing eye dog these days also Labs and Goldens for the most part GSD's make up 15%.
GSD's carry some baggage that you don't need in a service dog, not saying that can't do it.
If you want a dog that "looks" like a GSD without the "baggage" that can come with the real deal consider a Shilo or King Shepherd.
Personally if you can train little dogs, I don't think you'll have any problems with a "real" dog.
Assistance Dogs International : Dog Breeds & Behaviour