I don't live anywhere near you, but I do have a dirtbike (have for more than half my life).
I'd buy an old Honda XR200. This bike is quite possibly the most bulletproof on the face of the planet. Have a friend who's mechanically inclined go with you to look at it and make sure there's nothing majorly wrong with it.
After you feel comfortable on it, then you can move on to virtually any dirtbike, based on your needs (motocross, trails, etc).