I don't want to rain on your parade, but this does ring a few alarm bells for me - maybe unfairly, and it is clear that you have the very best of intentions. However, it does sound like this was a bit "on impulse". This is your 1st dog - you did not, I don't think, decide you wanted a SB, but there was one needing a home so you took her.
You are proud of your clean home. Dogs are dirty creatures, and in the winter, large furry dogs are truly awe inspiring in their constant muddiness. Any walk in the park/country will leave the legs and underbelly muddy and needing to be washed when you get home. The bigger the dog, the more hassle. I have a large breed so know what its like. She will want to walk out in the garden when it's wet and then come back in - usually when you are in the middle of something, and then you need to get a towel and wipe wet paws.
Also, this dog is 2yrsand from what you say, has not been trained, socialised, or even lived as a pet. There is a crucial period for puppies- the 1st 12 weeks when they need to be introduced to different situations, people, children ,animals. Once this period is gone, it then becomes quite hard to get a dog well socialised and to react well to unfamiliar situations. Do you know what this dog is like with other dogs? Or on the lead/off the lead? If she does not want to come in the house, it may be difficult to get her used it, or she may react badly if she feels confined. She has not been used to responding to people from what you say - as it seems they have not responded to her - chaining her up or just letting her wander about on her own. You could end up with a dog that is not the companion you envisaged. Yes, SBs generally have lovely natures but if not socialised from an early age they will still be affected. What is she like with the noisiness and unpredictability of children for example?
The Breed Association will be a good place to start for information - but please do think about the expense and commitment a dog will be - apologies if you have done this. Insurance will be expensive - the chances are this dog was from non -health tested parents as a responsible breeder would take back an unwanted dog. large breeds are prone to joint problems - Hip dysplasia, elbow dysplasia, osteochondrosis, not to mention heart conditions. These can be very expensive to treat. - operations for these conditions can be £6000 or more, so I would suggest getting pretty good cover. I would just mention that all breed associations would run or know of a breed rescue - often with people waiting for dogs who have experience of the breed, so if you feel this may be too much, you could contact them. The last thing this dog needs is someone to take her on just to let her go a few months on.
If you do keep her, then I wish you loads of luck, I just want you to know it may be difficult!