Oh that's lovely to hear, OP! So often these dogs that aren't 'perfect' are overlooked - so this had made me unbelievably happy!
Behaviour: From what I've found dogs with disabilities can be quite defensive (note I do not say reactive or aggressive). They're slower and less nimble their their able-bodied counterparts (and in this I include dogs with only three legs, blindness, deafness etc) and so that can make them nervous but not in a fearful way but more in a 'please be careful with me way' when they are in unfamiliar situations (primarily with new dogs because humans are, by nature, more careful with dogs with disabilities). If other dogs move to quickly, for example, they can get quite nervous of that.
Physical: It's worth making sure she is comfortable being picked up and handled - as I'm sure you know, it's very important for older dogs, but also for dogs with disabilities you want to be sure that when you are holding them or carrying them you're holing them correctly (because your grip will change because she does only have three legs)
Comfort: As she is older you may want to consider one (or two - depending on how big your downstairs is) beds suitable for older dogs. She'll need to put more pressure on her other joints to get up from the floor/bed so you'll want to ensure that transition is as smooth as possible and she has support. As PP said, keeping weight down is very important.
Exercise: Walks little and often. You know springers - you know they're often bonkers and that doesn't calm with age. But as with the above point, because of her disability she will put more pressure on her other legs and given her age, you won't want to overdo it. So I'd aim for three (if possible) shorter walks a day instead of lots of big ones.
Personality: The above being said, I've also found that older rescues (I foster) and dogs with disabilities are often the most loyal and devoted to their owners because they're so reliant on them.
As she gets older you can look at wheelchairs specifically to help her keep going out and to ease the pressure on her other legs.
I hope the above is helpful! 😊