In Peru only 3 airports officially have avgas: Trujillo, Lima and Pisco. I'd suggest to clear customs in Trujillo and avoid Lima because Lima is quite busy and takes a lot longer to get in and out of. There are a few private airfields near Lima like SPLX (owned by Master of the Sky) that you can use as well, but contact the owners first.
In Colombia Cartagena (SKCG) is a beautiful place to visit and airport has good service. Cali is also a good fuel stop but I've never left the airport there, so I don't know what the area is like.
Peru typically takes the longest to issue landing permits. During the southern hemisphere summer, the coastal weather in Peru is very nice. Inland is rainseason from December - April, so be extra careful if you're planning to fly over the Andes.