Ask user to check categories of an item and then display category-specific options

Problem

In my Web application I have a form in which the user must fill some information about a generic item, let's say a car. Between all the standard information (name, description etc) I am asking the user to check the categories of which the item belongs according to the his/her belief (e.g. "Sport Car", "Subcompact Car" etc).

Now based on the category checked I need to ask additional information to the user. For example if he/she checks "Sport Car" I need to show another form asking for "Sport Car"-related informations (e.g. top speed) whereas for a "Subcompact car" category the average consumption.

Possible Solution

Let the user fill the car form (with the categories checkboxes) then the system will store the car object in the database with its checked categories. After that, the user can specify the category-related information in a special page say "Advanced Options" which based on the categories of the object will show all the category-related form.

Still I am not convinced since I would like to make the category-related informations mandatory if the relative category is checked.

Any suggestions?