I flew a long distance to get two of mine, and would do it again in an instant, to get the right dog from a reputable breeder. It costs a lot to maintain a dog. If you cna't afford the up-front cost, what will you do when something goes wrong? Besides "normal" costs, food, supplies, vet, grooming, training, toys, etc.) Kodi had a sudden and severe illness when he was young. He ended up hospitalized for about 3 days, and it cost me $1800!!! Things happen.
If you want a Havanese, save your money until you can afford what it costs to bring a good one to the island. While most reputable breeders prefer not to ship, they do make exceptions. Havanese are sold back and forth between US and foreign breeders all the time. And for many of these puppies, "under the seat" is not an option due to country or airline regulations. It is not ideal, of course, but puppies, in general, DO survive just fine this way. I have a friend who is a breedere. Two of her dogs were imported from Europe and one from New Zealand. They all flew cargo, were well cared for on the trip, and arrived bouncy and happy. Inside the continental US and Canada, I would want to fly with the puppy. But I understand when there are circumstances that make this impossible. Can things go wrong? Yes. But they are rare.
That said, pretty much the only places that Havanese and Shih Tzus are similar are that they are both toy breed dogs with long coats. Havanese are much more energetic, need more companionship and more training/interaction to be happy. They also, in general, crave companionship much more. Shih Tzus are a bit more "cat like"... Not much into training, content for longer periods by themselves, more independent... nice pets, but very different temperamentally, and while I prefer Havanese, Tzus are easier for many people.
I also don't know where you got the idea that they tolerate heat better... The average Havanese will find the coolest floor and nap on it when it is hot. The breed may have originated in Cuba, but they don't tolerate being outside in really hot (and especially humid) weather any better than most breeds. That said, both Shih Tzus and Havanese will do fine in a warm climate given adequate relief from the heat with cool shade and/or AC.
It is true that Havanese do not have the health problems that brachycephalic breeds do.