What Bella said about breeders, my pup came from a terrible breeder and only cost £200. When the person who bought my pup had problems the breeder flatly refused to have him back and got very very offensive and threatening. This is how pup came to me
On the surface he seemed a great breeder, fantastic surroundings, clean kennels, healthy pups and mums, good natured dads, hip scoring, the lot but when it really mattered he just said "stuff you" (only much much more rudely)
They are bouncy, they are an awful lot of work when they are little and it does last a good 18 months. At about 1 yr you will start to see glimpses of the sane dog they will become but they come and go
With mine, he has to be excercised 2 or 3 times a day, for very short times because of his joints. We take him out and let him sniff about, play with other dogs etc but don't really walk him yet IYSWIM. In between those he needs stimulation, he will sleep for 30 mins ish then be up. If you leave him it takes about 40 minutes for he is running from the bottom end of the garden right through the house to the front door, barking his head off and cannonballing anything that moves. We have puppy training classes once a week so we have little practice sessions in the morning, afternoon and evening, different things each time, sometimes it is sti/stay, others it is let me play with your ears/check your teeth. This is more about keeping his mind occupied than him actually learning. After that he will sleep a bit, then he is up and off again so to head that madness off we will have a play session, find it, catch etc again 3 or 4 times a day. GSDs are very mouthy pups too and it can be unnerving for smaller children who are at a prety good head height for an adolecent GSD, even older dc can find it a bit much at times.
With all of that we manage to keep him just about sensible! But it has to be every day, there is no let up when the dc (or you) are ill and this kind of routine will have to continue for at least another year. It is pretty intense! To top it all off, it has to be said that our pup is very calm for a GSD, lots of others are lots worse than him!
I am sure that you have researched all this and know what you are getting yourself into but I thought an "insider" view may add some colour to your findings!