You would need a case that is slim and 'elegant'
most elderly people are insecure about computers and what is a reasonable price,
talk about what the person is getting, what it means to have a 'Linux computer'.
Even giving them an option for you to set it up for free, anything to make this seem legitimate and reasonable encourages a person to buy your product.
and for heavens sake, please don't install 'raspbian' and leave it be. if possible, overclock it and set it up for this person so it is set to go.
also include the keyboard and mouse, this makes them more likely to buy the Pi.
I'm sorry for rambling, but i like to go a bit far with my posts