wrote:

Aviation GPS's need periodic map updates. So what I'd ideally like is
rather low-end land/marine unit with a basic map (cities, highways,
shoreline, other topographic features) that has available software to
create my own waypoint database of airports and navaids to supplement
the base map.

The Garmin eTrex Legend is a small handheld that includes a reasonable
basemap (roughly the level of detail as state highway maps) and has a
PC interface cable for use with a wide variety of available software,
much of it free. It'll let you load in up to 1000 waypoints at a time.
Retails for around $140.