Jo - Looking at options on Amazon it seems like the Magellan eXplorist series and Garmin Montana series would be good options. Both are waterproof, use conventional batteries, have topographic maps (eXplorist comes with street as well), and big screens (4" on the Montana, 3" on the eXplorist). The models (and prices) within each series seem to vary based on whether you want a built-in camera (so you can geotag your pictures) or how much map data you want pre-loaded. If you want to blow all your hood insert sales money plus some go for the eXplorist 710 or Montana 650t - they both run right about $600 on Amazon. What I couldn't find is whether they display your lat/lon directly on the map, but you'd have to think that's a feature.

There are significantly cheaper options but usually the screen is smaller, they only work in the US, don't include street maps, or aren't waterproof.

Off subject: The best GPS I ever had was the factory one in my 2003 G35 Coupe. It had two features that are missing from most units: a "Find Along Route" option to show you all the search results within a short distance of your planned route, and a backup positioning system that uses the last GPS fix + speed data from the car + accelerometer data from several on-board gyros. You COULDN'T get lost in that car - f***-ing awesome.