We are looking at plans to build next spring. One of the companies that seem promising is Topsider Homes. Is there a representative from that company in Belize? How good are the builders there...Could they handle this type of building- Basically prefab with a few interior changes? How much per square foot to build prefab homes?

http://www.topsider.com/

Yes seen quite a few topsiders in the country over the years - some are over 20 years old.

Topsider homes are excellent in this environment. They are really well engineered and super-durable.
The "issue" is all about importing them because they are made of wood. There is no representative in Belize. You'd need to buy stateside and ship the kit. Check with a custom's broker to see if they can get the kit into the country.
The Tree House on Ambergris Caye and Nautical Inn in Placencia are Topsiders - not sure who put them together or what special talents they needed to do so.