Morning Rochester.. you turn up buy a ticket.. (for as many laps as you want).. and go play.. no pre-booking is required.. as long as they are open.. and those times vary as the NS is booked out for manufactures to do their testing .. you can play..
My last trip was in my main work car.. on the way back from Frankfurt.. and to be honest.. you only go as fast as you want.. but my car has a hard life and to be honest its time on the ring it was not as heavly treated as on some of the back roads in the UK .. which I know better.. lol..
In fact in the wet on the ring.. I felt like I was staring in driving Miss Daisy.. with everyone piling past me.. lol.
There are insurance comapnies for cover but i think someone once told me I would be looking at £350ish for the day..
check out www. nurburgring . org they have all the information you could want..