Do not believe the Highlander has NAV, was not an option when I was looking a few months back. You also said "not high end", a loaded Highlander gets close in cost .....big reason I went with the MDX.
Alpine makes the unit in MDX, and an Alpine can be added to any car for some cost. Nav units are about $1300 to 2000 new, and the display is additional. Pioneer, Clarion, and Eclipse as a minimum also make units. I had the Alpine CD Rom based Nav with an Alpine indash receiver in my old Explorer, and it was great.
As Rick mentioned, the Streetpilot III is great as well. I have it for my motorcycle and am happy, but it is not cheap and you only get 1 software region, unlocks are additional. Also slow compared to the Alpines and no inertial based guidance. On plus side though, works well and can be moved. Am happy I bought it. There is also a Garmin 5 (I think the # is right) available for much less than the SP III, but it is not a color display.
Several options, depending on cost, and they all work