I agree - if you already have a rulebook, you have set yourself on the right path.
I had one "false start" with a vehicle project before buying a rulebook. It was for a vehicle that I knew well, had good experience with, and had a vision for an LSR application. Fortunately, it didn't take long for me to figure out that in order to build the vehicle of my vision, I would never be competitive against the existing record, and it would cost me a fortune.
After getting a rulebook, reading it, re-reading it, re-reading it again and asking a bunch of questions, I found a class and vehicle combination where I thought I could be competitive and afford to spend the $$$ on the project.
Of course, the cost of the project blossomed, and it has not yet run on the salt, but I am ready for the next "dry spell" at Bonneville.
Steve.