Here are a couple of suggestions from performance paddler who is rather new (3.5 years) to the building scene:
1. Read the kayak design section of Nick's book, the "Strip Build Kayak." Ok, read the whole book but focus on the design section while you are choosing which boat to build.
2. Do anything you can to get more paddling time in a wide variety of boats. Find a good outfitter in your area and go talk to them. Many will let you demo and/or rent boats. You may also want to make the drive down to Annapolis to one of the monthly CLC kayak demos. This will help you narrow down the features that you are looking for.
3. Recognize that your first build will probably not be the "perfect" boat for you. Once you build the first boat and paddle it for a while, you will have better idea what you really want.
4. No one boat is great at everything. If you have room, put together a fleet with a range of targeted uses. A golfer carries 14 clubs designed for a variety of different shots, so why can't a kayaker have a couple (or seven) different boats designed for different uses?
5. As a first time builder, choose a design from one of the better designers and stick to the design. Don't over think it, just build it the way he says to.