If A is to remain in the existing house, can the existing house be adapted to make it easier on the person with illness in particular, like putting in a downstairs bathroom or a Stannah type lift to upstairs (seeing it as an aid to keep you mobile and functioning, even happily living, in your existing house rather than a symbol of defeat), and any other changes like perhaps a workstation to allow the ill person to do potential work from home etc? Is there space to make the living space work now and potentially into the future if you need to adapt - space for a walker/wheelchair, replace sofa with more upright individual chairs with arms, that sort of thing?
I am not trying to suggest these as an expectation of needing them, from what you initially said - but that thinking about these things as part of any moves or not may help the decision as any house may need to be adapted.
And also that such adaptations can not only help the person affected, but actually really make a positive difference if the person accepts them - as they can help the person to stay at home, and be comfortable, for much longer than otherwise. And also anyone either living with that person, or acting in a caring capacity, will also find it a lot easier.
I had a DGM who had to go into a home at the end of her life (no one living locally to support her as she needed and state care was limited to 3 visits of 15 minutes per day even though much more was actually necessary), as she needed more support than available at home - but she had nursed DGF at home for 10 years after a stroke with an adapted bathroom (mostly just grab rails and a seat in the bathroom to sit there when needed and a seat in the bath to allow DGF to sit while showering rather than fear falling over), converting the outside shed into a toilet accessible from downstairs, and an extra bannistairs rail (so 1 on both sides) on the stairs. DGF had been pretty badly affected but was able to move around slowly at home, and had a walker for on the flat.
I had a DGM who stayed at home until the end, going from spry at 91 to bedbound at 92. She had more support at home (1 DD living at home still, 2 DDs living locally and a DS living locally too who all were able to do a lot more on a daily basis, as well as the funds to pay for private care rather than depending on state resources). So she had a stannah lift installed, a decent folding and lightweight wheelchair was hired allowing her to get out and about, bathroom was made easy for her to use etc.
And I have a DUncleIL who is wheelchair bound now (in mid-70s, he was walking on 2 false legs until prolonged bout of dealing with cancer, and consequent being bedbound, meant he lost the mobility on his legs). He has a reasonably active life - self-contained flat on ILs house (for 40+ years), which in recent years has had an adapted bathroom and a couple of ramps put in to replace steps to the doors but he still cooks, studies, writes etc there and looks after himself reasonably well with some additional support but not loads.
Its as much about getting any necessary amendments thought about in advance and either put in place or at least knowing the options to do them when needed, which has made a big difference to those people staying at home as long as they all have. And all in their own surroundings that they have or had lived in for decades.
Is it worth getting an occupational therapist or an architect who deals with adaptations to have a look at the existing house to consider what changes might be needed, in your own particular circumstances, and if they would be possible to the existing house?
And it may also give you an idea of what possible things to look out for in any house searching you do - either in terms of things already adapted or specific layouts that may work better for you as a couple, or things that you can see would be possible to adapt relatively easily if they become necessary in the future.