or has it become so invasive it affects the whole leg
Generally, the older a dog
is, the more risks surrounding surgery and recovery time is longer. That is a fact, and the vet will probably say the same. There is a chance either she won't wake up, or will have a hard time recovering from an amputation.
That's the worst case scenario, for which you should be prepared.
However, that said, I am a great believer in tough old dogs! I have seen a few, that's why.