These threads are indeed a very good start! You could find similar threads on any car forum with enough research.
I cannot advise on rental companies, as your research online will be as good as mine. Sorry to be of no help here but I have always gone in my own car, a friend's car, or a random stranger's car! I did once go in a rental, but that was a regular Sixt car rental (BMW 1 series), which I advise you do NOT do. So, just do your normal research into those rental companies and you will find something reasonable. Trust me, everyone is trying to make a buck from the Ring so you have a lot of options!
A video game might do something to get you somewhat familiar with the corners, but do not even remotely think you know all but the most familiar sights from a game! Every turn looks the same until you've been around 10-15 times. Forza 3/4 model it the best from what I've seen, but don't use a video game for anything more than something to get you excited to drive on it.
I am not the best driver out there, by far, and I have spun out on the Ring before. Be aware, you do have to pay for damages to grass, gravel, etc. I simply spit gravel onto the track, and it cost me 50 Euro. Just something to keep in mind.
With all this said, do not approach it like you would a normal track day. There is virtually no runoff and little room for mistakes. Take it easy, enjoy your experience, and don't worry about that VW Polo passing you :biggrin: (but don't let that school bus pass you! and yes, you may see out-of-place vehicles on the Ring). Leave very aggressive driving to those familiar with the Ring! So, definitely try to hire or simply talk your way into a ride with a good local driver for the real thrill. That is my best advice.