I'm a contractor and I do a lot of remodels for people getting older with concerns about mobility. One of my specialties is creating a walk in shower that does not have a lip or a door. You could get into it with a walker or a chair, or just walk in without worrying about tripping. Not knowing your finances, or the situation in your house, just based on what you have said, putting money into the current house would be a mistake.
My thinking is that you need an easy to get into house, which means no stairs anywhere. You need wide doors, halls and bathrooms. Space is what allows you to get to where you want to be if you are in a chair or using assistance.
I think that a metal shop building would be the most cost effective way to do this. Hire a reputable company to erect the building with all the plumbing done for the bathrooms and kitchen. Get the architect to work with standard sized buildings, something like 40x60. Then once the building is done, you cane either hire a General Contractor to finish off the interior, or hire each of the trades yourself to get it done. I'm currently working with a guy who has a 30x40 building that he is doing this with. One bedroom, one bathroom and a very open floor of kitchen, dining and living areas. You could drive a golf cart around in there!!!
Eddie