A tough decision. With the age of your cat and the fact that it sounds like her quality of life has really deteriorated, you have to weigh it out. Spend the money, fix one or two fixable problems, and help her deal with the rest, knowing that she's nearing the end of her life. Or just put her down. Is she suffering? That's up to you to decide. If she's eating and her appetite is still good that's a good thing. Maybe the other stuff isn't too bad.
One of our cats was 17 when we put him down. Our daughter grew up with him, he was one year older than her. He started to have issues with his spine and hips when he was about 15. We faced the same dilemma as you. He was a member of the family and our daughter loved him dearly. We just couldn't put him down. They gave him a steroids, which helped for a time. Later they wanted to do x-rays and possible surgery. Finally he could no longer walk more than a few steps. Point is, we waited too long. He was suffering and we should have put him down a year before we did. Although I can reason that we were talked into keeping him going, it was our emotions that kept us from putting him down.
My thoughts are with you. This is not an easy thing.