You don't have any possibility for deleting a Basemap on this device. Basemap is not available for modification by user!
You may loading any maps with Mapsource to you device without doubt in crash system! Simple combine maps with limit in 22-23Mb (24Mb only in manual) and download to device. In any case you may switch through menu which map be show in device - basemap or detailed maps above basemap or only only detailed maps without basemap.
About Garmin web site and U.S. TOPO 24K maps for East, West, ... I assume what you must buy separate a device and the DVD with maps for loading with Mapsource.