First off, welcome to the forum! This is a fantastic resource so spend some time looking around and reading reviews, etc and soon enough you'll have a ton of ideas!
I'm not sure when the best time to get married in the Caribbean (if there even is a "best" time) is but I've always thought to stay away from hurricane season and school vacation/spring break seasons. We're getting married in January in Jamaica where the average temp is 80 degrees. We didn't pick January for any specific reason, it just so happened to work out that way. Of course, this is a subjective opinion because since we live in the northeast, we could encounter a nor'easter on the day we leave just as easily as a hurricane could hit the island we're at!
If you're trying to figure out prices and which location would be best for you, check out Sandals website and run some rates to get an idea what price range each resort is in. When deciding on my location, I took travel time into consideration. Initially I wanted to get married in Antigua but it would have taken twice as long to get there as Jamaica so I had to compromise. I didn't want to spend the better part of 2 days traveling, nor did I want that for our guests either.
Do you have a TA? They might be able to help steer you in the right direction.
You will get mixed answers about making a reservation at one of the restaurants. It's really hard to get a definite answer until you arrive at the resort. It seems to go by availability and booking a butler level room may help get you a better table. The WC's in Miami (who you will be in contact with up until you travel) will tell you they cannot guarantee a large group together. You might be ok if you only have 10 guests (we will have 10 and are taking the chance to get a table together) but if you are going to have more like 20 people, you might want to consider the private reception. Once you book and are put in contact with the WC, they will provide a booklet of the options. Basically you can choose anything from a 30 minute cocktail hour for $15/pp all the way up to a full 3 hour reception for $75/pp.
Sorry to be so long winded. I've learned a lot in the past few months just by reading about other brides experiences, on here and on the Facebook page (Fans Who Love Sandals Weddingmoons). Sandals isn't as popular as some of the other resorts on this board so sometimes the info is hard to come by!