What are the most important parts to upgrade to lightweight parts? Which ones save the most weight, or increase the performance the most?
Flywheel, engine block, chassis, wheels and a couple more I can't think of
Any exterior stuff, or are they mostly interior stuff?
I surly hope by saying interior stuff you mean the chassis and engine. Part of the exterior stuff would be the chassis and wheels, you could also replace the hood with carbon fiber or fiberglass