Prepare to be disappointed.

I wanted the same thing. It doesn't quite exist. You can get close though.

I have a Garmin eTrex Vista Hcx ($200 or so on Amazon). I also paid nearly $100 for the maps (may be optional). I draw my routes with RideWithGPS.com. I download a GPX track file. I import that into my Garmin software (Basecamp), then upload it to the GPS. That gives me a line on the GPS screen to follow, but doesn't give me turn-by-turn directions.

I tried downloading the GPX of the route to get turn-by-turn, but that rarely works. I sort-of works out in the country where there are few roads. It doesn't work at all in the city.

No matter what you buy, it'll be a learning experience with a dash of frustration.