Carolyn,
I don’t think that you want to require people with questions to jump through hoops to post or as you said, you may discourage questions. Not all questions require specific vehicle information. If specific vehicle information is needed, then potential respondents can easily ask for that with a few kind words. Some get a little cranky about it but those that do may need to take a break away from the computer.
Posting with inadequate input for respondents is common with questions in any subject area.