Take a look at the OsmAnd app. It's an app that uses downloaded open source maps for navigation instead of getting maps over the data connection in real time. I have a Garmin so don't use the app, but from what I heard it may be what you're looking for.

As far as your unlimited data plan, I'm with AT&T and they've kept my unlimited data plan grandfathered in over a number of phone upgrades even though they no longer offer an unlimited plan.