Motorsport is one area where I believe that the body can make a difference. However, only you will know if you've reached the limit of your current equipment and that it is it that is holding you back.
The decision comes down to how much you want to spend. Buying a used 7D with maybe a grip isn't going to give you much change out of £1000. It is probably Canon's best all round camera and can certainly hold its own with motorsport. I always found the AF to be at least as good as my 1D mk II. However, its AF system is way more complicated to set up to your way of shooting than the 40D and because of the pixel density, it is very unforgiving of poor technique. As you should from the spectator side of the fence, it retains the 1.6 crop which is handy in providing effective reach and it has a huge amount of pixels meaning you can crop quite aggressively into an image and still retain a lot of pixels. You need to nail the exposure with it as well to ensure you keep noise levels under control.
There are good condition low mileage 1D mk IIn's out there that will give you a reasonable amount of change from your £1000. This camera is still very, very capable despite its age. The AF performance is rock solid, and image quality is also right up there. Technology has moved on though, so it lacks the bells and whistles of the newer cameras. Only you can decide if those things are important to you. There is a certain magic associated with the 1 series cameras as well, so be warned that once you've used one you'll more than likely be hooked forever lol!
I've rambled on for long enough now. Don't hesitate to drop me a line if there's anything specific you want to ask.
Cheers,
Simon.