D2 car system is free as well as most of the commonly used add-ons. All thanks to Eight Blinker and brittylicious hard work taking their time to create and develop system for the community without any monetary gains but passion and giving to the people.
Great D2 modding tutorials made by Todd Squall.
Loads of tips and tricks as well as processes of how certain parts get modded or scripted. Timestamps in the description to cut to the part you're after.