Welcome and congratulations! If you are looking for some nice villas, I would check out either homeway.com or vrbo.com. They have homes for rent all over the world. I found one for my family when we went to Costa Rica on vacation and the house was incredible. I think that it might be helpful to also make a list of things that you might need to keep in mind. When I have to rent a house, I have to get one that is mainly one level as some of my family aren't as mobile as others. You might want to think about your guests needs as well. If all people are able to get around easily, Costa Rica is very nice, but I would fly into Liberia.
Good luck on your planning!