I would make it controlled by voice. Just need to figure out a way to tell the user what the system can do. Computer, help me shop for medicin and hold up a box infront of the screen with what you use today. This should be simple. If you make it to complex like it is today then elder people will not use the computer.
Basically a personal assistent/guide driven OS. Start with rethink the experience you would have if you went to the mall and have a human assistent with a big screen to show you everything.