Hey guys - as someone who actually lives in Belize, I really feel like I need to jump in here. First, Caridust, yes they sell 4 wheel drive vehicles here all over the place. Don't see ATV's as much, but you can get them here as well (duty is the same as any vehicle I believe).
But this is the thing - I've never had to put my truck in 4 wheel drive mode the whole time I've lived here. The roads are not as bad as people make them out to be if you live in the already developed parts of Belize. If you are looking to buy property that is under development or hasn't been built yet, then yes, the roads are horrible because more than likely they've not built them yet.
In addition, the Belize government has very little money, so they almost never pay for the roads - the developer or residents need to do that, and usually it's the dead last thing that gets done.
If you're gonna buy farm land and work it, I could see how an ATV would be nice, or if you live up north on the island of Ambergris Caye, where the roads do get flooded in the rainy season, it would come in handy. But last year when I lived on the island we managed with our golf cart just fine - never got stuck.
Get a reliable Toyota 4runner w/ 4 wheel drive option and you will be set anywhere in Belize. If you don't plan on buying in a new development, you'll definitely be fine.
Also, Mwwin15, jumping and hoping for the best is not a good idea in Belize. You should take it very slow, visit several times, move and live here for 6 months to a year to make sure you like it, and only then would I recommend buying anything here. I hope my perspective helps.