So what is the latest on this issue, what cheap as chips perfectly functional bike specific sat nav would one buy today in 2017. I'd be looking for a bright screen as unlikely to bother with bluetooth. Unlikely to fiddle with the buttons on the move so little or no buttons suits me. Use an old TomTom in the car and that works just fine for me.
The three big advantages of a bike sat nav are
1) it is waterproof
2) it will Bluetooth sound to your helmet-mounted BT device.
3) it is compatible with route planning software to allow you to get from A to E , via B, C &D.
Your could sort out 1) with a waterproof pouch 2) You don't need 3) even the cheapest car Satnavs are compatible with Basecamp (and other) route planning software.
If I was in your position I would go with a car Satnav in a waterproof pouch or look for a second-hand Garmin 340LM or equivalent TomTom, depending on your budget.