Will this be your first dog?
Are you buying her? If so how much are they asking?
Are you being given a pedigree with her?
White is a major fault in GSD's. They cannot be shown. If you are just looking for a 'pet' then of course this does not matter to you.
I would be interested why they are rehoming her. Sometimes breeders 'run a dog on' until they are six months to see how they develop, then decide whether to keep the dog or not. Sometimes peoples circumstances do change and they have to rehome the dog. But beware, people do not always tell you the real reason (especially if it is bad).
GSD's can suffer from HD (a low score say 6/8 is good a high score, say 30/40 is bad) and if you are given a pedigree then the hip scores for the parents should be on it. This DOES NOT guarantee good hips though.
GSD's can also suffer from epilepsy, haemaphilia and a digestive problem called EPI (they cannot digest fat of any sort - even chicken skin), if the dog looks very thin be cautious.
Go with your head NOT your heart! Don't 'fall in love' or feel sorry for the dog. Remember hopefully this dog is going to be with you for the next 10 or more years (your 6 year old will be 16!).
To be honest you would be better visiting a good rescue centre like the Blue Cross, or getting in touch with a GSD breed rescue, they have more dogs looking for homes and can match each dog to the people rehoming it.
One GSD rescue is
http://www.biggsd.com/ if you check out this web site it will give you an idea of what to look for with GSD's.