I just had a guy help me buy two cars in Santiago, a Suburban and an Astro Van. Vehicles are expensive here and used ones break down often. I recommend renting.
But if you want help buying one then I can recommend my guy Agustin. He is honest, affordable, and gets the job done.
Start by surfing to Chile Autos.
And if you need Agustin to help you with paper work and securing your RUT then shoot me a message.
Also, I am the owner of Pucon Kayak Hostel. Should you come to Pucon and need affordable lodging or kayaks to rent then I would be happy to help you.
Pucon Kayak Hostel