There are two sets of rules, one for cars with license plates, and one for those without.
If you have plates your car needs to steer, brake and run without any leaks. And you need at least a three point seat belt. Simpson and other aftermarket stuff is fine but it needs to be in good shape. Slicks is also ok. (don't tell anyone that I said that
)
To use your car on the track you need to buy an "license" , basically a insurance as you normal one is void on the track. This cost about 44 Euro.
Cars without plates are considered race cars and need to hold a race car license, that are issued by your "national race association". To drive a race car you also need a full personal racing license, that you also get from the "national race association".
If you have any other questions, PM me or call me at +4792495271
BR
Jon