We got a Garmin eTrex Vista and the European Cities maps for our summer-long backpacking trip. Be warned that the accuracy of these GPS toys is very iffy in cities with lots of tall buildings or narrow streets, or even in outdoor areas with lots of tree cover. To make a long story short, it was very useful a few times, but most of the time it was extra weight (local, free street maps were handier). The plan was to sell it all back on eBay when we came, which is what we did.
I'm looking forward to the next generation of GPS which will work much better in tight areas (read about this in Scientific American). Anyway, if your dad doesn't go for month-long trips, you can get away with a GPS unit lacking a large amount of memory, because he won't need to load huge numbers of map sections. So, you can get him a moderately priced one and and in a good topographics map set from which he can load whatever area he'll be visiting next.
Good luck!!! GPS units are fun toys.