Do a load of research on the web (and here is a good start) then do a trip for several months minimum, before committing and stay in a couple of different spots to see how you like things in each even if you think you know where you want to be. We did the same when we first arrived and from our research thought Placencia was our likely destination. Once there we realised San Pedro was a better fit for us and we only realised this as we had stayed in San Pedro on the same trip.

An imported boat will cost loads to get here and incur Duty at around 50% of it's value. A big start up cost.

Never assume anything. If you had friends move to Belize three years ago and you lost touch it is more likely they left than stayed. I would guess a third of those that move make it into year 3-5. Perhaps less.