if they don't need all those fancy features, just go with an entry-level device from a known manufacturer. Garmin (nuvi), Tomtom, Magellan all make nice devices.
What you might want to look into is what maps a certain brand uses. There are a few companies that supply maps to virtually all manufacturers. Some are more european oriented, some more american oriented. Another thing to look for is the possibility to connect an external antenna. Especially when the car it will be used in has a heat repelling windshield. Usually there is a little cutout to let GPS signals pass through, but its often at a very impractical place like behind a rearview mirror or very low above the dash, which would make it virtually impossible to mount the gps there. An external antenna will give you a bit more freedom in the placement of the gps device.