I am thinking that you want this to be your forever house. Using Universal Design principles, the house would be made accessible for all people.

Here are some places to look for more ideas and information.
Please take from this whatever is helpful. Best wishes.

Houzz
1.) Go to Houzz Ideabooks and enter "Universal Design" in the search box.
2.) Go to Houzz.com/Handicap-accessible
3.) Go to Houzz Design Dilemma and enter "handicapped" in the search box.

Universal Design
1.) www.universaldesignstyle.com
2.) www.schaefferconstruction.com specializes in Universal Design

Handicap Access
1.) The Americans with Disabilities Act (ADA) site has specific measurements for wheelchair turning radius, angle of ramps, handrails, everything.
www.ada.gov/adast94.pdf
www.ada.gov/
2.) ADA Ramps: ADA Code 4.8

Aging-in-Place
1.) Consult with a Certified Aging-in-Place Specialist. You can find one through the nahb.org website.
2.) Go to Houzz Professionals and contact Brickwood Builders. They are Certified Aging-in-Place Specialists. (For you other Houzz Professionals who are CAPS, please add your name to this list!)

What is the size of the existing bathroom?