We had a similar good driving experience in Peru a few years ago. A local who helped arrange our trip told us we might get stopped by police expecting bribes. He told us the standard amount that would set us on our way, or that we could fight it and contact our embassy if necessary -- up to us. We only had one stop where it seemed like there was an issue. But our Spanish was so poor as we tried to understand what was being said to us that he apparently decided it was not a good use of time and sighed and waved us on. The only other thing I would mention, probably obvious: carry toilet paper, none of the restrooms along the road will have any.