If you have a smartphone or tablet that runs Android don't even bother thinking about buying a sat nav.
Just download "OSMAND" for free, but you are limited to 10 map file down loads, some countries such as France and Germany are divided into regions and need more than 10 map files to cover the whole country.
Or download Osmand+ which cost less than a fiver (£4.99 actually) and unlock a package of extra features.
https://en.wikipedia.org/wiki/OsmAnd
https://play.google.com/store/…smand.plus
The detail of Open Street Maps makes TomTom look like maps drawn by primary school kids.