Sorry in advance if this is a bit long winded.  I had 2 cats, one is normal but the other one was a bit of a physcopath.  When we got him as a kitten he loved my GSD.  After my GSD died the cat absolutely HATED dogs.  He would actually attack them, even if they were visiting the farm next door. We realised we could not get another dog until this cat had gone.  He recently died (aged 15). My other cat has met a few dogs and while not actually liking them, has never been too bothered by them.  Any puppies she has met have been given a swipe on the nose and then ignored.
My dilema is that I know someone who has 2 long haired GSD's aged 11 months. They are brothers and while she now realises that she should never have got 2 together unfortunately when she went to see them, they were the last 2 and she ended up taking both.  She has worked VERY hard on socialising them properly and they are both fantastic natured dogs.  Great with people and other dogs.Sadly she just cannot cope with both at once.  One of them especially is quite hard to train.  He is too interested in his brother and seems to have got it in his head that his brother can get trained and he will just play about. She now realises that she should only have got one and honestly beleives that the best thing for both the dogs is to rehome one.  They have started playing very roughly with each other.
As I was going to start looking for a long haired GSD pup I said that I would have the dog that needs training.  I have got the time to give him individual attention, help at a local dog training class and the person who runs the class is very qualified and would be able to give advice if needed.
We  brought the dog to our house and let him have a good sniff around and settle down.. I then carried in the cat.  It is the first time I have ever heard the cat really growl.  SHe was absolutely terrified.  The dog was on a lead and was allowed to come up to her slowly and she of course swiped his nose (not unexpected).  Now he is a VERY big dog, but what was worrying is that he did not back off at all.  He was not aggresive but desperate to get to her.  The owner did say that he chases and catches rabbits and will not give up once he has got the scent of one.  The cat was too frightened to run away until I stood inbetween them.  The dog went home again but the cat would not come back in the room until the next day and would not let me near her.  She is nearly 14 years old now. My own GSD bit one of my cats once.  My Fault.  I fed them in seperate rooms and had not shut the door properly.  One bite and the cat had a dislocated jaw, haemoraged behind the nose and had a ring of teeth marks around it's neck. Not something I would ever want to happen again.
I would really love to have this dog but I am worried that even if we could stop the dog from getting the cat in the house, he would catch her outside. 
I know she would be fine with a puppy.

I have always had cats and dogs but have never introduced a full grown dog (I know he is still a puppy, but he is the size of a horse), to a full grown cat.

Has anyone any suggestions/advice (a magic wand would be handy).  Even if it is to say 'Forget it, it's not fair to the cat'.
Any help would be greatly appreciated. 

To be honest I would forget the dog & look for a younger one or one that is ok with cats

A late friend of mine ended up with 10+ rescue cats & her old Affie, when the Affie died the cats were bereft & spent hours calling for him. She asked our rescue for a cat proof rescue GSD & our lovely lady provided one in the form of an adult long coat who had been literally thrown ot of a car on the M62 & picked up by a passing driver. Our rescue lady had a lot of rescue cats herself & Sefton(as he ended up being called)bless him was lovely with them allowing them to sleep on him & even groom him ! Off he when to my friend & lived out the rest of his life surronded by his feline mates who took to him straightaway & who loved him on sight

I would not take on this young dog he would I feel be better as a sole pet until his problems & training were sorted out

i think you know your self taking on this dog would be a big mistake,expecially with him not been cat friendly.it wouldnt be fair on the cat to bring this dog to live with you!you would be better getting a puppy as there more likely to get on! x

thanks for the reply's.  I think your right, I probably knew in my heart it would be better to get a pup, but was just hoping against hope that someone had some wonderful idea, because with some one to one training I think he will be an even more fantastic dog.