What are some optional things that I might not need?

Obviously, a WRC car has everything. And at the other end of the spectrum, there are cars that are bare bones. There are lots of examples of people who have started building a rally car, but ended up never actually racing because they thought they also needed everything before entering their first event. The best approach is to look at a very minimum list and get yourself to the rally. Finish that rally. Then come back and add new toys for the next rally.

Things you don't need at your first rally:

• Rally computer or odometer. Route books in the USA are well written and you can manage easily with the car's stock odometer.
• Rally light pod. You'll survive without one, just back off on the night stages.
• Anything made of carbon fiber. Get that first event out of the way.
• Brake ducts. You won't be using the brakes as heavily as you imagine.
• Lexan windows. The eight pounds you save won't make a difference at your first rally.
• Seam welded chassis. Save that for your second rally car.
• Hydraulic Handbrake. A lot of work for only 1 or 2 hairpin turns where it could be used.
• Skid plate, maybe. Certain rally events, like Sandblast Rally or HyperFest, could easily be managed without them.