There are lots of possibilities, but we would need to know more about your needs. What type of fuel do you have available (gasoline, diesel, propane, natural gas). I believe Panama is 120/240V split-phase like the U.S., so that makes it easier find generators.
If you have an idea of what you need to run during an outage (your critical loads), add up the wattage (or the amperage) and that will determine the size of the generator. Or, are you wanting to run the entire house just as if on grid power? Also, would you want electric start or pull start?
As for transfer switches, you might consider reading thru this thread (I don't know what your local codes are)...
who would be into this ??
www.powerequipmentforum.com
Let us know. There are lots of people on this forum that can provide ideas for you to consider.