Artemis, from someone who made a decision to drive internationally for the first time this past November, please do yourselves a favor and rent a car. I was glad I did. I was always concerned about driving in another country primarily because I considered that I didn't know where I was going while in other countries. Well, I finally decided if I wanted to know, I needed to start driving myself and learn how to get where I needed to go, rather than rely on the taxis.
Additionally, when I considered the cost of a taxi versus what it would cost to rent a vehicle, it made sense for me economically.
I also found that driving on the North Coast is no more difficult than in many US cities. I admit that you will see some unusual things along the way, but I assure you it will make for some interesting and fun table talk. Be as careful and defensive in your driving as you would anywhere else.
And, finally, if I recall correctly, Prestige Car Rentals does provide service in Santiago, among a host of other cities/towns. I have used them In POP, so I cannot comment on the quality of service in other places, but I can certainly vouch for their service in POP. I first used them in November, again this past December, and I will continue to use them until I decided to purchase a car there. Also, after first being made aware of them on this site I did some Internet research, and actually found that they are just as competitive in their rates as most of the other rental companies in POP. There is always a less expensive route to take but, I believe you pretty much get what you pay for.
In any event, enjoy your trip.
p.s. I assure you that I am not a paid employee, just a satisfied customer.