we rented a 4x4 twice to go from bz city to cayo & back. that is all i can help with. the roads go from fine to awful, depending where u go. the gas is horreduously expensive (i think around $4 US). if you break down in the middle of nowhere, good luck. miles & miles of nothing between help (and gas stations). no vehicles allowed on the cayes without prior approval. do not know what kind of accomodations you are looking for or where. in any case, you will be sure to have an adventure.
i believe Lan Sluder has books & websites on these topics. do a google search on his name, or try belizefirst.com or something. anyone???