Ron,
Belize has very few all-inclusive style resorts. But an increasing number of hotels are offering a food package which you can select if you like. But in most areas we have many good restaurants to pick from. So it’s a shame to eat all meals at one resort when you can mix them up and try both local and some higher end restaurants.
I am not familiar with the best way to get from Montreal to Belize, since that is a very specific area and I don’t know expats who travel from Montreal to Belize. Major carriers AA, United, Delta, USAir all come here. I assume those airlines fly out of Montreal. But your best bet, when looking for a flight is to do a current search on Kayak to look at what options are available. Airline routes and prices change constantly. There are several air flight search companies on line, but Kayak is one I find helpful.
You do not mention where you plan to stay, the quality of accommodations, or what your lifestyle is like. We do have a list of hotels in the Belize Manual, with their prices. And keep in mind that you’ll pay over $75/day for a car rental, if on the mainland. Those will be your two biggest expenses, so start there. IN terms of food, you can eat a Belizean style meal for under $5 in most parts of Belize. But you can also easily spend $50/person at a high end restaurant.