The least amount of rules needed for safe exiting racing would be a goal. I have made a few cars that were not safe, but they were very exciting!
Here are some ideas:
No exposed lead, no sharp corners not part of original body
weights above wheels must be inside body
body can be modified (stretched as example)to cover weights and fit base
some bases (trucks for example) have a higher clearance, under carriage weights OK if not exposed lead (cover with tape etc.)