Welcome and Congrats!
So far planning a DW has been the best decision we have made! Choosing your location will the the hardest part. Once that part is done it is all down to the little decisions. Make sure you check out Del Sol's Wiki to help you with your decision! There are LOADS of slide shows of weddings at many different resorts all over the Riviera Maya. It will give you a good look at the resorts and what weddings look like there. I could spend HOURS looking through all the beautiful pictures!
I chose based on a few things: cost (for us), accessibility ($ and time for people to get there), and the wedding packages/perks the resort had to offer.
We look at EVERY possibility for a location, and Mexico ended up being the most affordable for us. Once we decided on where it was trying to choose a resort. That is where this website came in handy! No only did looking through the pictures help, reading reviews on here helped too! We ended up going with Moon Palace because of 1) they have a Catholic Chapel and 2) the group booking perks (ie book so many guests and you get free events and nights free)!
The only issue we really had to consider was the safety in Mexico. I am not worried about it at all, but I know there are worriers in the family, so I had to be prepared to either have info ready to show them it was OK, or realize some people wouldn't come. So far, it sounds like most people are coming!
We wanted to keep it fairly small, so we invited only family and close friends. At this point its hard to say how many will go. We have a lot of "maybes". But yes, I think it could shrink considerably. We sent out our STDs 1 yr in advance to give people plenty of time to figure it out. We will be requesting people send their final RSVP (that will be given in our invites) by EARLY Dec at the latest. Those that don't we will have to call. All the rooms get released 60 prior to the wedding, so we will need to know before then. If people want to book last minute they can, but it won't be guaranteed they will be with the rest of us or get the group rate....but that will not be something I worry about...it will have been their choice!
Spring 2012 doesn't sound too terrible! It honestly is up to you and how much time you think you want to give yourself and your guests to prepare. I knew I needed a year to get all the things I wanted done without too much stress, and I wanted people to be able to have plenty of time to save. Put the feelers out and see what kind of feedback you get from people. I think you could do it, but how much of a time crunch you can handle is up to you. Some people work better under pressure!
Good luck and happy planning! You will find LOTS of info on here to help you!