I would suggest Garmin Zumo 550. Zumo is essentially the same unit as Nuvi, but it is specifically designed for motorcycles. As such, it is a bit more rugged than the Nuvis. Although the unit is designed for motorbikers, it comes with a car mount and works very well as an in-car Sat Nav.
If you then arm yourself with premium membership and GSAK, you've got an almost perfect solution for paperless caching. I've found over 500 caches using mine, and only bought a handheld unit to tackle walks that take more than 4-6 hours (the battery life in my Zumo).
Happy caching.