Hi there,
To fix the wheels going into the ground open up your VehicleWheel blueprints (front and rear wheels) and increase the radius of your tires. The best way is to measure the wheels in your modeling program and input that value directly, this should fix that issue.
For the collision issue, Mike Skolones from NVIDIA is currently investigating a couple of issues to do with the vehicle setup in UE4, he has not found the cause of this yet, but you can see his progress in this thread on the forums. It is a known problem though, hopefully he can find the cause soon.
Hope that helps! 