There is one more often over-looked factor. That's how much time you spend on your boat. Get your kit as soon as possible so you can begin to design and build it. You may even be able to get some things done before you buy your kit. If you have a block to cut, plan to borrow or schedule now. Make some of your deck-o-rations. Get your craft supplies. You don't want to be left waiting for people and tools to become available. If you can, it is better to finish building early. Waiting to the last minute won't allow you the luxury of putting the final touches on that great paint job. You may have little time to practice blowing your boat in a tub of water.
Here is a rough time line to help you plan your progress. Change it around to fit the date of your race and when you can get your kit.