Hi Sheila,
The CostaRicaGuide Map that you have is the one we usually recommend because it is fairly updated and differentiates between roads that are paved and those that you will need 4×4 on. If you are getting a GPS with the car rental and use this map to make sure you are staying on the main roads, you should be fine.
One other option, if you are smart phone savvy, is to download the Waze App on your phone. This App has the most updated roads and traffic conditions in real time. The rental car company we recommend can set you up with a WiFi stick for the car for about the same price as a GPS and you can then use your phone with Wifi to use the Waze App as a GPS…as well as check email, Facebook, look at our website, etc. 🙂