we were debating between Koralas (Los Altos) and Karahe/Costa Verde's Event Center or El Avion. We ended up with the event center and it was gorgeous, but may be too big for 40 guests. Koralas would probably be just right for that size. It's modern and pretty, but the views aren't as striking (which doesn't matter much after the sun goes down anyway). I can't speak about Los Altos's beach b/c we didn't visit, but Karahe's is a stunning backdrop and has really reasonable pricing (agree with last poster and wouldn't recommend staying at Karahes). Los Altos condos are really pretty and can be affordable if you stay on a lower floor or share the space. They also have a great happy hour!
We stayed at Casa Paraiso in La Reserva (a little further up the road but still accessible) and it was one of the highlights of the trip. There may even be enough space to have the wedding right there, but not sure if that's ideal. We used Sylvia from Costa Rica Paradise Wedding and were really happy with all her help. She's reasonably priced and we might have been able to plan a wedding without her, but we probably would've paid more and had a ton more headaches - she was especially helpful b/c we didn't visit beforehand and she really helped with every decision and all the logistics. The flowers alone that she helped get couldn't have been more perfect! We didn't visit Arenas Del Mar while we there and none of our guests stayed at the hotel, but that could be a good option for a smaller wedding. Punto de Vista is also an incredible villa. We didn't see it, but it looks beautiful online and becomes more reasonable if everyone stays there. We visited the Parador and are glad we didn't have things there - just not our style at all - definitely prefer Karolas or Costa Verde's venues over Parador's. Although if you're looking for a more resort-like hotel with ambiance inspired by ancient Rome, the Parador's your place. We ate at the Gaia restaurant and would probably choose Karolas over that location as well. Overall we were fine with our guests being in different hotels and villas - nothing was too far away (we rented a car) and there were plenty of opportunities for us all to get together (welcome BBQ, ziplining trip, rafting trip, wedding, and brunch). Just fyi, it's not easy to visit guest in Los Altos or the Parador (esp the Parador), so this may be worth noting if you have guests who will be staying in other locations. Everyone who had a villa was really happy - Casa Antonio, Casa Paraiso, Casa Feliz, Casa Capuchin, and Casa Miramar (all different levels of luxury and various locations - Alex from Vacation Villas Costa Rica helped my parents, my brother and a group of our friends get really good rates). We also loved Casa Romantica from the pics on the internet but it was booked during our wedding weekend, so no one we knew was able to rent the place.
Good luck with everything! I was so overwhelmed at this time last year planning for our August '12 wedding, but we and our guests had an amazing time - everything was perfect and don't regret a single decision (although we - or I - agonized over each one!!)