Gs-com and Fultec are also good options in Belize City. Id check out those two places, plus the one Diane recommended and secure the best price that way. Also check the yellow pages in the phone book under "computers" and call the places to compare best prices.
I'd check Xtreme Geeks if looking in San Pedro. It will cost more, but you'll save money on transport and time.
Most monitors made recently will have DVI, so unless it's really old you should be alright.