When you go the smartphone route, some considerations. 1) If riding in the wet, you might need a waterproof or resistant case, factor in that cost. 2) Battery life might not be as good as a dedicated GPS, but easily fixed with a battery stick. 3) Screen may be hard to read, you're call if it's tolerable, and only an issue if mounting to the handlebar, and factor in a mount for the phone PLUS waterproof case if installing on the h-bar (dedicated units come with h-bar mounts).

For your needs though I would think a cheap Android smartphone, plus case, plus mount, would be cheaper then a good GPS. You then get to experiment as to which software works best for navigation - Google Maps or RWGPS and the app on the phone.