I have no problems with using the car either with or without a roof (I have both types of roof too!), while wearing a racing helmet.
There's lots of headroom and even though I have long legs I don't need to use the full travel of the seat - larger people than me fit just fine. You will have no problems once in the car.
Getting in and out is another question - most people have problems at first (it is something you learn) - the taller you are the harder it is to learn, but now I know how to do it, I can get out in about 3 seconds. It takes most people about 20 seconds before they have learned. I don't think you'll have any problems once you know how to do it.