If your parents are on board and you have the funds, why not wait, save your money, and buy a well-bred pup from a reputable breeder?
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So if you are serious about getting a GSD, do a little research first and check out some reputable breeders in your area. There is a difference!
It really, really, really is in your best interest to wait for a dog right now. Something I'd like to point out, that you might insist up and down isn't going to be the case... I hate to sound rude, but if you're 15 years old, this is going to be your PARENTS' dog. You might have the friend with her, you might even pay the initial $150 for her, and if you're posting here you're at least mildly concerned on making sure she gets what she needs, so I'm sure you'll help out where you can and likely take her to at least a puppy class. But you have to think about your schedule and your future. You're probably a Freshman, right? Sophomore at the oldest, so that's at least a few more years of school. Whether or not you have activities going on, school takes a huge chunk of your time just with classwork alone, let alone if you're talking about getting a job to help fund things for your dog. Then what about hanging out with your friends? There's more time you're going to be away. What are your parents' schedules like? A puppy can only hold their bladders a short while, so I'm guessing your parents are going to have to take up the slack with the dog while you're at school, although you can always help exercise her before and after, and before bed.
Then there's college. If you're going to a community college, you'll have the same schedule, for the most part. Away for class quite a bit, then off at work. If you go away to school, you'll have to leave your dog for months on end. It is HIGHLY unlikely you'll find housing that will allow a dog. Most schools have requirements that new students live on campus for their first year, and I've yet to hear of any on campus housing that allows pets other than a small tank of fish. So, then, you're talking about even more time the dog is going to be away from you, at home.
By the time you're out of college and ready to have your own place, I'm willing to bet your parents aren't going to be willing to give her up, or if they are, it might be a really rough transition for her. She will definitely know you and love you, but she will be comfortable at home and chances are (because I've seen this a MILLION times over working with dogs, and also anecdotally with my in-laws) you will all come to the decision that she should stay at home.
What I'm getting at with all of this, is that it is a HUGE decision to make for your family because you happen to have a friend selling puppies. If your FAMILY wants to make the decision to get a dog, it's time to get everyone together and have some serious conversations about what is and isn't right for your family. Be HONEST about everyone's time commitments, financial commitments, and what sort of personalities you would all mesh with. Discuss how you all feel about training and how getting a new dog would affect your current dog, because he doesn't deserve to have a puppy as big or bigger than he is bothering him if he's not the playful type. The last thing you want is to get a dog that not everyone is willing to put in the effort for, especially if you only just now are looking into things like this forum where people can give you advice on the breed. It's all well and good to SAY that you are all going to have the time and want to help out, but it's really important to do research to find out just how much of a commitment it really is, especially with a difficult breed like GSDs.
Look in to getting this book:
Living with Kids and Dogs . . . Without Losing Your Mind: Colleen Pelar
Even though it is geared a lot towards people with younger kids, there is still a TON of information about what to look for to pick the dog that's right for your family, and there ARE sections in it about families with older kids like yourself. It's a FANTASTIC book, and I think everyone trying to decide on a new dog should own it!
After all that decision making is all said and done, you're much better off waiting with this pup. It's a better investment to save up and buy from a breeder that can help you select the right puppy for your family, like others have suggested, or you may even decide that you'd like to get a rescue. (Just a side note, we adopted our husky at 5 years old, he already came to us crate trained with a small bit of obedience training, and he is GLUED to us!) It's REALLY REALLY hard turning down a puppy when they're RIGHT in your face and you have the money, trust me, I know!! My husband's been having to reign me in for years! haha! But learn from the many other people who have made similar mistakes and just use this as a jumping off point to learn more and get the dog that is going to be best for you guys.
Good luck with everything, whatever you end up doing! Keep us all posted!