I used a company called rave-tv at the Brands Hatch festival 2 years on the run. (I wanted video evidence I had raced there for when they trash the track)
Race TV had some pretty impressive kit and where very cheap for a 1 weekend deal. This is an option if you wanted to hire a camera for a one of event. If you wanted to buy, I suggest you give Nick at Race-TV a ring, he should be able to help you pick the right kit for your car and budget. I can assure you the 2 videos of the 2 years I got where almost digital quality ! there where EXCELLENT. They also had pretty good sound too.
If you want to see the videos to judge the quality - let me know.
Nick is a nice and helpful guy and the website is
http://www.race-tv.com
Give him a call.
Matt.