I don’t drive here, so I’ve never used a Garmin, but it seems to work fine for friends who do. I don’t know how accurate it would be on little country roads, but it should be perfectly fine for getting through cities and on major highways.
You can buy a sim card for your phone here, but I’m not sure if you can purchase data for a short period of time or if you need to purchase a plan. Either way I wouldn’t recommend going that route. Cell service can be spotty in parts of the country and phone data can also be slow. It wouldn’t be something I’d want to rely on to get me from point A to point B in a new country.