That sounds like a great gig.
As for mics - well, the Sennheiser MKH series are your best bet in high humidity situations. Don't know what kind of budget you have available or if you can rent mics, but perhaps a pair of MKH 8040s would suit you well - they'd be great for both ambience and music recording.
The NTG3 should perform very well in high humidity as well.
I'd be a bit concerned about the DR680. The build quality is so-so and I really wonder how it would hold up in a demanding environment such as the Amazon. At the very least I would look into a good bag solution that would keep the recorder as unexposed to the environment as possible. Mind, the DR680 might have no problems whatsoever, but I think it's a risk.
Maybe consider getting a Sound Devices preamp/mixer to go with your D50 as a backup solution that is built more solidly than the DR680 in case the Tascam does break down.
//edit I just remembered I recently bought this album, which consists of a very long field recording in the amazon: http://fieldcraftrecords.com/album/a-deep-forest-creek You might want to contact that guy (David Michael) through the contact form with some questions? He used different gear than you would take with you but he might have some good tips.