Well, I can make some recommendations. I also can tell you to expect problems no matter what. Hand-held radios are incredibly unreliable things聴the second someone walks to the back side of a hill, he or she might as well be on the back side of the moon. Add to that the issues that your people will be facing, such as rain and humidity.
Icom IC-M88 Radio
IC-M88 RadioSo for starters, I聮d probably dismiss the radios using the FRS and GMRS frequencies, which have a range of two and ten miles, respectively (under ideal conditions). They might be ok for around town or camp. Motorola聮s T6500R radio (motorola.com) sells for about $50 a pair.
Better to get a real hand-held radio, I think. Icom聮s IC-M88 (icomamerica.com), for instance, is designed for marine use and can withstand lots of punishment. It聮s pretty powerful, with five watts of output, and compact. The battery is rechargeable, although there is an optional AA battery pack. The downside: Cost is about $500 each (maybe less if you shop around). Kenwood聮s TK2140 ($700; kenwoodusa.com) has similar specs but in a larger form, so there聮s more room for goodies such as programmable keys.
The other thing to keep in mind is that these radios use radio bands licensed in North America. You might well find that in Guyana those same frequencies are used by, I dunno, the local radio station. So check with Guyanese officials (perhaps in their U.S. embassy) about the utility of American hand-held radios in their country.
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