I have a Garmin 76CSx, and it is waterproof, floats, can be used in hiking, driving (with optional map), and uses AA batteries that you can buy anywhere. Since you may be in areas where electricity is not always available to charge your batteries, I would highly recommend a unit that uses AA batteries.
Yes, there are people who makes their own maps for their GPS units, and then share the maps with the public. Some are in well known websites, others are hard to find. Here are a few map sites:
http://gpstracklog.typepad.com/gps_trac ... for-y.html
As for being careful on the trip, a friend of mine, and his son, took a motorcycle trip down south, with the tip of South America being the final destination. They picked up 2 new bikes in TX. But, before they got to the Panama Canal, his son dumped his bike and bloke his leg and a few ribs. They finally found some town's people who had a pickup. The locals made a cast using cardboard, and he and his son rode on the back of a pickup for 4 hours (with no strong pain drugs) on a bumpy dirt road, to get to a local rundown shack of a hospital.
My friend then arranged with a local office export/custom office to get his two bikes shipped back to the U.S. After paying $2000 for fees and shipping, they flew home, and waited for the bikes. After emailing the export/custom agent, the agent said they needed additional $2000 for some additional customs fees and decontamination stuff. Well, after a total of $4000, they never heard from the agent again.
Have a fun and careful trip.