In all honesty, those keywords are too broad. And if you want a reputable SEO firm, they will charge you a pretty penny (myself included). Narrow your focus on specific car models and the word "forum", "message board", "board" etc. Write articles and find relevant blogs and websites that you can do link exchanges with. Become active in the car forum community and promote your site that way. You may have to bootstrap and do the time-consuming promotion yourself. This is largely why many SEO (that are good anyway) companies charge as much as they do--there are no shortcuts to proper SEO.

Create videos and viral blog posts that will bring visitors to the site as well, leverage the power of YouTube, Vimeo, Viddler, etc. Create podcasts. You need to syndicate that blog content so that it can spread easily on multiple different platforms. For popular blogs the vast majority of threads are NOT READ ON THE BLOG.

Go to Flippa.com and buy established sites related to cars and send them to your forum. That is probably cheaper and faster than SEO (in the short term). You mentioned you're creating a forum site, I really hope you have other ideas besides ads to support the site. I can tell you that ad revenue this early in the game won't come close to covering the costs (whether its time or money). You will need to consider CPA and affiliate programs in the immediate term. Don't forget the power you have in the e-mail list you build, think about what kinds of offers your audience would be interested in. Do not spam them with constant offers though.