I've seen a LOT of sites like this in my time, and no matter what anyone says, people will pay for answers because they believe if they pay, somehow its going to work better then asking on a forum.
Your missing a few key elements to this. I'll give you a quick overview, but if you want my help, please contact me... info in siggy
You need to build trust with the user, there is none there. "Professional?" Add in some mechanics certification images and best yet, add a video of a mechanic talking. "show" the person who they will be working with.
Second, what kind of mechanical questions can I ask? Is this someone that can help me fix my HVAC or someone that is going to help me take the steering wheel off my jet fighter? There are a lot of mechanics, if you are targeting the automotive industry, notate that.
Lastly, before you begin an AdWords campaign, you need to set your site up to convert better. Right now, you only going to get maybe 2% of the population, what about the other 98%? Just going to say "forgot them!" There is where my expertise comes into play, but I don't want to type out 20 pages of text if your not interested.
Also, with AdWords, make sure you know your ROI figures. If your net revenue is $14.95 and with a 2% (probably a bit high to be honest) conversion rate, you can't really spend more than $.30 CPC as it does not appear that most people will renew each month.
Like I said, if you need any help, feel free to reach out to me