You will love visiting our country, Caldera is a convenient port to arrive. To answer your questions:
Yes there are rental car options from Caldera/Puntarenas. You can arrange to have a representative from the rental agency meet you at the docks. I would consider using a private transfer when you arrive and then picking up your rental car after you arrive at your first destination. This is a way you can see what our roads are like before committing to driving yourself. Please stay away from a rental car company called ECONOMY. This is a company with several complaints, most others are reputable.
Where are you staying while you are here?
Have a fun trip!