I'm not a pro builder either, but the biggest mistake I have seen is there is no plan for their build. I think they see a car and go "that's what I want". They start putting things together then realize, where am I going to put the battery, gas tank, and wiring? Soon realizing they have built themselves into a corner. When you start cutting up your project because you didn't plan all this out in advance it gets discouraging. I think in the early stages of hot rodding you should use the KISS theory.
Ken