Use the NSX a few more times to really see if "really" caught the bug or not. If you can purchase a cheap track car that is partially already modified for track use, I would recommend that. I purchased a used MR2 that had a seat, rollbar, and decent brakes installed and did a ton of track events in it after doing 8 or so events in the NSX. After you are comfortable and confident with your skill level, you'll probably know the next progression naturally based on the level of your interest. As for me, I've recently aquired a dedicated race car and am taking the next "hit" off the go-fast-crack pipe.