If you're a noob going into this sport EXPECTING to roll your car hard enough to require a roll cage, you probably shouldn't try it in the first place. No offense.
This "sport" isn't nearly as dangerous as you make it out to be if you properly mitigate the risks involved. Start with an organizations that's been lauded for its long history of safety, work your way through until you've gained enough knowledge to be dangerous, then you can choose to go to the lapping days and drive "flat-out" to your heart's content. Then you can make a determination of whether or not you want to risk your life every weekend, or if you want to walk away from the inevitable.
And I'll bet at that point your car will be so far removed from being able to be recognized as a stock M3 that the point of a permanent cage or a bolt-in one won't even cross your mind.
The hell I don't! LISTEN, KID! I've been hearing that crap ever since I was at UCLA. I'm out there busting my buns every night! Tell your old man to drag Walton and Lanier up and down the court for 48 minutes!