If you are thinking of a German Shepherd please research research research into the breeders, there are some incredibly bad ones about and you could end up with a very sick dog in a short space or time or one with social issues such as aggression or anxiety, these idiots give the breed a bad reputation and should be put out of business for the sake of the animals welfare.
I read a very interesting, and rather scathing article in
this magazine the other day, from the woman who showed up the inbreeding of dogs for Crufts etc.
I would highly recommend you read it before buying, it's the current issue.
German Shepherd puppies are expensive - hundreds of pounds, if you buy cheap the chances are you are buying from a puppy farm or as a previous poster has mentioned, one with a bad hip score which will mean they will have severe problems later in life.
German Shepherds are working dogs, not show dogs or lap dogs - if they have a severe sloping back they will suffer problems later in life (far better to find the 'Anglicised' Alsation version which shouldn't have as much of a slope and is bred for working and not the show ring.
You generally find two types of Shepherd...the lazy I'll do it later variety (such as my first one) and the mad up for absolutely anything constantly variety (the two I have now), both types are wonderful in their way and will be loyal and playful to the last.
They are incredibly intelligent and do need things to do, it's no use just leaving them a ball and going out for hours, you need to be able to commit to them, give them interesting games - finding treats, puzzle games, agility games, hours of playing tug etc.
Could I also suggest a rescue German Shepherd? We have one puppy from a breeder and one rescue now and he's the most loving dog, we had to travel a way to get him but it was completely worth it, I suggest these sites to try:
BIG GSD - very helpful site
Lancashire GSD Rescue
German Shepherd Rescue - a site run by a rather 'passionate' person, their heart is in the right place if you can get past the strong wording.
Dogs Blog - a fantastic site, where you can search by all sorts of variables from type of dog to the area it comes from.
*edit* if you are taking it to structured training the dog should do brilliantly, it's great that you are thinking of doing that!
One thing that I don't think has been mentioned, you need to get your current dog to meet the new potential several times on neutral ground (neither dogs territory) before choosing one, otherwise you may end up with two stressed dogs, a wrecked house and miserable owners!