Quite an 'open ended' question but ...
Check out Nurburgring.org ... There is also a post on here with some more details, you may have to search using 'Nurburgring'.
Key things, IMHO are ...
Medical insurance,
European medical card,
European breakdown cover,
Notify insurance company of travelling abroad,
be clear about your insurance cover/situation with regards medical and car given the place you are going,
All the usual kit you need to trabvel abroad i.e. triangles, bulbs, first aid, maps etc.
Also be aware of the rules regarding speeding and vehicle confiscation in Europe ...
I have made use of European breakdown cover and medical insurance abroad and I WOULD NOT recommend you skip or skimp on it !!
By the way when are you going exactly ? I want to do a Ring trip this year and was considering Easter ...
Martin