Well, I can make some recommendations. I also can tell you to expect problems no matter what. Hand-held radios are incredibly unreliable thingsthe 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.
So for starters, Id 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. Motorolas T6500R radio (motorola.com) sells for about $50 a pair.
Better to get a real hand-held radio, I think. Icoms IC-M88 (icomamerica.com), for instance, is designed for marine use and can withstand lots of punishment. Its 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). Kenwoods TK2140 ($700; kenwoodusa.com) has similar specs but in a larger form, so theres 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.
The Gear Guy reports from 2007 Winter Outdoor Retailer, the bi-annual gearapalooza in Salt Lake City. Check out his top picks for gear to watch in 2007.