The coins should be captive in some way when they are delivered to the output, so they don't spill on the floor. The device would be used in shops, pubs and post offices and on buses, so there may not always be a place to put the device on a flat surface. The device should try to reject foreign coins, buttons and washers that a user could mistake for valid coins. There should be a minimum of parts to go wrong. Too many devices for disabled people cost the earth.
Price is important.
People who have these problems due to age may be pensioners so they won't have much money. It is no good to assume these will be bought by charities for people, they may want to buy them themselves.