Hey Pete,
Thank you for your prompt reply.
What I need to have in my site is:
1- When you publish an ad for a NEW car, you get a form with a lot of details including HP, torque, Suspension, etc....
2- When you publish an ad for a USED car, you get a form with some not all of the details in form no.1
3- When you publish and ad for RENT car, you get a form like form no.2, but a RENT period is in it.

All previous share same categories (which are car makes and models in my case), so what is the best solution you think?

Thank you in advance for your support.