Dr. Moab, I am certainly no expert on all the Garmin Nuvi models but my understanding is that most of them will do Topo maps just fine. Just load the maps you want in Mapsource and then copy onto a SD or microSD card (whichever your particular Nuvi uses). You will not be able to customize the Nuvi the way you can a handheld in as far as what you have visible on the screen at any one time. If you have to have a fairly large screen and all the features of a handheld then you need the Montana. But that's big bucks!
For the past couple years I have used a Nuvi 5000 as my primary vehicle GPS in my expedition vehicles. 5.2 inch screen with twice the resolution of any other Nuvi and for my ancient eyes it's a godsend. It also has an external antenna connection. To keep the sun from washing out the screen I mount the Nuvi up against the roof of the vehicle and in this position it has trouble seeing satellites so the external magnet mount antenna goes up on the roof. Here are some shots of the screen on mine. These are all the GPSfiledepot maps that you can download for free. I also use Garmin's 24k West (Washington, Oregon, and Nevada) maps but don't have any photos of them. They are pretty much the same but have a different color scheme.
The Nevada GPSfiledepot maps show private land as white and public as dark green; highways are orange, contour lines are brown and secondary roads are gray. Your track (which cannot be turned off) will show as a bright blue. But they will show most of the mines, ranches, peaks, and even wells/windmills. Having driven these same roads for the past 40+ years it is interesting to me to see what is out there that I didn't know about before, until I saw them on the GPS!
The below was taken in the old (1860s) Hamilton mining district west of Ely, Nevada and each of the pick/shovel emblems indicate the location of another ancient mine:
This one was taken on Christmas day returning from Oregon, where US 95 drops off the mountain and crosses the Owyhee River. The colors of the Oregon GPSfiledepot map aren't nearly as dark as the Nevada map and are easier to read!
If you want to see your coordinates or elevation or which satellites the GPS is using, you have to touch the menu button and go to a different page of the display....
Hope this information helps!