I like the Garmin products and feel they are the best. TomTom and Magellan are starting to copy them as far as widescreen modes etc.
These two units are pricey but I've seen many OTR drivers with them and they love it to death.
StreetPilot® 7200 StreetPilot® 7500
I use a Nuvi for my peddle route. Damn thing is awesome! I still carry a map book with me to write stuff down. I also use it in my personal vehicle to find other stuff.
Stay away from the local stores like BestBuy or CircuitCity, Walmarts, Sams's, Costco etc. They still sell all GPS units close to retail prices.
Check this link out: GPSdiscount.com ..They are Authorized to sell Garmin products so you can be assured that they are
not black market junk. Very affordable stuff in here.
Keep us updated on what you end up getting your father.
Remember:
If it wasn't for your dad...you wouldn't be born. 
Get him the best!