to you OP, hope you will find a way to work around your illnesses and also that you can stop beating yourself up about the impact on your family.
"The best laid schemes o' mice an' men / Gang aft agley"...
I think the space and layout you are looking at would be fine for your family.
Can you make a storage cupboard under the stairs? A coat cupboard just inside the front door?
Can you take some space from the rather big living room and make a walk through pantry/ butler's pantry that would have built in drawers and cabinets, or on both sides? You wouldn't have to demolish the existing wall between the kitchen and living room, just build an extra one and in the intervening space put in the butler's pantry. You could take between two and three feet off the length of the living room and still have a nice room. Pictures of butler pantries usually feature a counter but you could do without that, just find a unit or have one built that has deep shelves on top and drawers on the bottom, or cupboards with sliding doors.
Extend kitchen cabinets right to the ceiling so you can store stuff you rarely use up there.
The fourth bedroom as a big walk in closet/ bedding storage/ winter coat and footwear storage in summer is a good idea.
Get the attic finished or at least boards for a floor and the roof lined so you can put stuff like suitcases, fake Christmas tree, etc up there.
Put in hinged banquette seats in the family/diningroom for storage of all sorts of odds and ends. Put in floor to ceiling cabinets on the sunroom party wall (get the sunroom option if affordable).
You can get bedroom storage/bed units where the beds are raised over the storage, like bunks but with cupboards and drawers underneath.
www.pinterest.com/pin/286400857533943052/?lp=true
Go on Pinterest and you will find many, many ingenious ideas for making the most of kitchen cupboards, and storage in general.