Hopefully, you will already know the different kinds of people that are in your community and the different sorts of needs they will have. After all, you've been selling cars in that community for quite a while.
So, what you should do is to come prepared to talk about the needs of any demographic since there's no way to know who will show up at this talk. If you do surveys and all that, you're probably wasting your time because there's no way to know whether the people you survey will resemble the people who show up.
What you should do is have various slides made up to cover the different kinds of needs people have. Then, at the start, you give basic advice. From there, you ask for specific questions and go from there. Or you could ask things like "how many people need to know about buying vs. leasing?" Or "how many people have questions about financing?"