I would check out used/cheap PNDs. You get a minimal enclosure with enough room for a pcb and battery (you might even reuse the battery and maybe the charging circuit if possible). I bet you can find some older ones with 3-or-so inch resistive screens for under 40 bucks - probably 10 at a garage sale. As long as the enclosue, screen and battery are ok (you may not even want the battery) it should be a safe bet.
Pawn shops (ahem) tend to have lots of these as well.
Granted you are just hoping you can gracefully separate the components you want from the PND guts - it might be worth checking to see which PNDs have some open source projects around them - the component specs are probably known and you can find the datasheet - maybe even in English :D.