Seniors are great, but yes they do require more maintenance vs. a younger dog.
The stairs are okay, but I wouldn't choose a dog that you know has arthritis - Tyson has arthritis, and he can't do stairs. I mean he could... but it wouldn't be smart because it can flare up the arthritis. This is why we bought him a ramp for the truck.
The costs medically will all depend on what health issues they have, which if you are getting from a rescue, all of that is disclosed up front. Tyson is as healthy as a horse, except for his arthritis. So our extra expenses for him are for meds, joint supplements, and laser treatments. Nothing else really, we do run blood work on him bi-annually due to his age just to make sure nothing is off, and if so we can catch it early!
Older dogs are more laid back, and chill. I don't think Fezzik will be too much per say, could even help give the older dog some more pep in their step
Bullies are living longer & longer these days - with excellent care of course. I wouldn't worry so much about the depression part, try to focus on the time now. (That is something I am trying to work on myself)
The one thing to remember is that older/senior dogs are set in their ways. They are even more stubborn than younger ones, and just like us humans they will have good days & bad days... they are a little more moody.... but they have so so so much love to give. I think once you have one, you'll see how much love they are capable of, especially since they are not the first picks in rescues. They deserve just as much love as a younger pup in my opinion, and give you back double love.