Emfiver put a lot of solid info and I agree with most of what he said.
I was going to rent an E9x M3 from Big Garage who canceled with me less than 12 hours before my rental because of "insurance issues." So I scrambled to find a short notice rental from Ringfreaks, ended up with a stripped out race prepped 320i. (Still a bmw right?) Granted its my only experience with a ring rental company but I'd recommend them. Super accommodating and fairly priced.
Between it being my first time on the greenhell, and it pouring rain, and the wife not taking kindly to the rear end stepping out it really was for the best. I'm sure I would have gotten a little "over zealous" with a 8250 redline. Also no one, even private party for obscene amounts of money has an E60 to rent
Can't really blame them though. I can't imagine what ringtaxi spent on E60 consumables back in the day.
Ringfreaks really did their best to put the fear of god in you as far as insurance works. From what I understand, unless you get your own dedicated insurance for your particular car on your particular day, at the particular track, if you end up causing any damage to anything you will pay through the nose out of pocket. Got to pay to play and all that, but I can't say it didn't help temper my right foot either. If you want to play with exotic rentals there were more than a few which wanted deep 5 figure holds on credit cards just to get the keys.
The only thing I really want to disagree with Emfiver is concentrating on your lines regarding tourist drives only. For tourist drives, the nordschleife is treated like a regular road, meaning you pass on the left you don't drive on the left. If you are taking the "correct racing line" and end up wrecking the ring marshals will hold you at fault. It was super hard for me cope with at first. Not that everyone obeys it but the rules are different for right of way on track. I'm sure whoever you rent from will go over it all. Again only applies to tourist drives.
I certainly didn't set any lap records but also didn't wreck or run off track and was able to finish 5 laps in the pouring rain. There's not much more than 2 hours open for them on the vast majority of slots and most are in the evening so don't plan on a whole track day persey for the tourist drives.
If I had the time, as in multiple days there, I would have loved to do a ring taxi so I could experience it without have to worry about liability and all that while also seeing what a real professional can do compared to me.
I'd also pony up the cash for a good instructor for all of my laps to get the most out my time on tarmac. There's something to be said for jumping in feet first in the deep end all by yourself, but for the sake of getting the most out of your time driving, instruction is seriously worth it.
Lastly I had to check my ego hard, there are people who go there weekly if not daily. I got passed by such a wide variety cars that at first felt super embarrassing. Focus on you and the track and enjoy it. It's hard to remember when your out there that it's not a "race."
I can't recommend it enough and can't wait till I can get back.