Sounds like a bad idea to me. I don't think you can count on your 300/month job and the internship to cover the costs associated with a loan and keeping up with the 135, not to mention your student debt and the costs of being a student. Yes, it is a newer car, but still out of warranty and the N54 isn't exactly the model of reliability. Selling one used BMW and buying another used BMW to save money on upkeep doesn't really make sense to me. Especially when all the money you make for the next year and a half or so is going to be used to pay for the second car.
What happens when the new car has some issues? Sounds like your parents can help you if you are in a bind, but that doesn't show them that you are being responsible. If I am you I either keep the e46 and continue to maintain it (sounds like with 6k in recent work it should be in perfect condition) or buy yourself something more reliable that will not require as much maintenance.
I would not have wanted to worry about a loan through my parents for a new BMW when I was in college.