It's an interesting question, but there are a lot of additional details I'd want to consider:
- Will you be staying in hotels or out in the wilderness? Can you assume daily access to AC power, or will be out in the woods?
- How often will you be near "civilization" to charge up batteries, connect to wifi? Can you assume regular access to a Starbucks, McDonald's or other wifi-friendly location?
- How big a factor is weight? Are you tossing stuff in checked luggage or carrying it on your back?
- How do you feel about carrying multiple devices?
- How much of your valuable vacation time are you willing to spend downloading and converting books?
- If you want to minimize weight and dependency on AC, I'd pick the eink reader with the broadest selection that suits your needs. No guarantees you'll find everything, but you can find plenty. Be sure you can actually buy books overseas. You still need wifi to buy books.
- If you want maximum access to ebooks, and are OK with maybe a little more weight, a general-purpose tablet with all of major reading apps pre-loaded and authorized should work. The worst case scenario is finding a site (e.g. Entitle Books - recommended) that lets you download an .acsm file. These can be opened directly in Aldiko on an Android device and, assuming you have momentary wifi access, the actual DRM-encumbered .epub downloaded. An Onyx Boox T68 might give a good mix of portability, flexibility and battery life. You still need wifi to buy books.
- If you want to be ready for anything, a combination of eink reader for battery life along with a laptop/tablet capable of running conversion software (e.g. Calibre) works, but makes regular access to power a bigger deal. You still need wifi to download books, unless you have a device with an RJ-45 Ethernet port.
A big part of the challenge seems to be network access. For that, a suitable phone with an international data plan and hotspot functionality might be simplest assuming you can get coverage where you plan to be.
For power, you can use a variety of high-capacity battery packs. The smaller your device's requirements, the further you can get between charges. There are options like solar panels or hand cranks if you'll really be out in the wilds.