I've just bought a Garmin Nuvi for the missus... not sure it can run other software, I know you can load pictures, mp3s and audiobooks but I've not seen anyway of installing software. I'll take a better look when she gets back off her trip.
But, it's another vote for the PDA & Tomtom/Memory-map combo.
I'd probably get something like a windows mobile PDA with built in GPS (there's one for less than 200 squid at Expansys.com for example) and then see if I could ... get hold of the software.
Upsides are:
It actually does mapping for on and off-road
It's fairly small compared to a full on Tomtom (510 etc)
You can play games on it while waiting to be recovered
Downsides:
Not as straightforward to use as a dedicated unit, underlying OS issues.
Screens usually are'nt as readable in direct sunlight, depends on the unit.
Expense if you decide to buy the software.