I am a wedding planner here and I strongly recommend hiring a planner as finding reliable vendors, speaking the language, making sure things run smoothly are priceless. All the planners I know of offer full-service coordination only. As we have 8 years experience and have worked with almost all the vendors in the country, we have narrowed down our vendor list to only a select few who have proven themselves to be reliable and professional. For this reason, we do not take on day-of coordination as the few times we've had brides bring in their own vendors, it's been less than stellar results.
While a planner may seem like an 'expense', they will save you a lot of headaches as they know the area, they know the vendors, they know what does/doesn't work in various locations and will ultimately save you money. And if you're having a wedding at a villa, remember that you will need to bring in rentals (tables, chairs, glassware, etc.) have extra cleaning staff, extra staff to move the furniture, etc. Being a beach town, Manuel Antonio has very limited services (ie no rental shop, only 1 flower shop that is very basic) so many items have to come in from San Jose.
Costa Rica is an amazing place for a wedding but it's still a 'developing' country and industry - some vendors do not have websites and a few don't even have email!
A planner will take care of everything so you can relax and enjoy your time here, rather than running about, wondering where the wedding cake is or if the florist has set up the flowers for the ceremony.