Hi Eduard,
We don't have any offices in that area and I have never been there, but I can answer some of your questions in a general way as far as property and construction in Costa Rica goes.
As to a builder, you should look for someone already working in the area and visit houses they have built. Talk to the owner(s) and see what their experience was.
An engineer or architect will be able to make your estimates for development. Your location is fairly remote, so someone already working in that area would be a good choice. Possibly a large office from another area could work, as the initial plans would be done in the main office - but they might be able to spare a person to supervise once development begins.
As to the other questions, those depend on your objectives for development - tourism, vacation homes, retirees, quintas (mini farms) etc. It sounds like you want to subdivide, so you will have to check with the city government for that area (Nicoya/Santa Cruz?) to see what is possible for your land.
Here is a good link from INVU with some of the documents developers need and where they get them and who has to receive them:
http://www.invu.go.cr/urbana.htm Also - I would be interested to know more about your purchase, who your realtor was and who the seller was. Also I'm not sure what you mean by
how is this area?