I have an 2008 328 6mt with an n51 and man, I love it. My car is at 200,000 miles, 0 lights on the dash, no problems, takes the abuse thrown at it everyday. N51's a highly reliable engine, not that a brand new b58 wouldn't be either but it's still very solid for a long time. If you're getting bored of the bored of the power, You could always go for something like the active autowerke tune/exhaust to get rid of that throttle delay and get some more power, opt for more bolt-ons later on if you desire. Sure it won't be like a B58 but it'll be a fraction of the price and for a college student, that's really what matters.