Chris,
The Legend has an 8Mb limit on upload/download, so you should have no problem with storage of local topo maps. And you can upload/download new ones any time.
You WILL need the Garmin maps, which do cost extra. The gps will come with a reasonable basemap showing all the local hiways and some mainroads, the lakes, rivers, railways, etc (though the map is a bit rough).

For my needs, I wanted all the room I could get inside the small etrex package, so I went Vista.
As Ray notes, not all maps work with all gps brands, so most serious types choose the maps first, then the gps unit that they need. And if the maps run on a laptop, then forget the gps, all you need is a small receiver unit to attach to the laptop. With laptop prices sooo cheap now, you can often get a better package just by adding a receiver to a cheap laptop. It can be hard to find a gps unit with a color screen at 1024x768 pixels and a high speed processor that will also do: wireless internet, spreadsheets, play dvd movies, mp3 files, download and hold a ton of dig camera images, burn cds and dvds, use cheap memory, and run all of microsoft's crap!

With the more expensive gps units, the unit itself becomes the "value", whereas with a cheap laptop, it's only the data and maps the now have "value". You can easily make a backup copy of the whole laptop to an external harddrive, and if the laptop dies/fails/gets lost/etc, you would only need to copy all the backup data to another cheap laptop.