Been there, done that.
Good luck with your course, I finished there in June '08 having just done the Nat Diploma. You'll have to work hard in the First Diploma group because most of them **** around and will try to drag you down. Twenty odd would start the year but by the end only a handful were left. The Nat Dip won't teach you anything about motorsport, only about the basics of a car. Get yourself out and about working for teams gaining experience while you are there, you'll learn more out working on racedays then you will in the classroom. Can't say anything about the FD because I didn't do it, but from a few people I know are still doing it it's pretty tough, especially the maths aspect of the course.
It's good to aim as high as possible, but mechanics are always needed at lower levels. I'm happy working where I am, off to Zolder on wednesday, went to Spa last year it's all good fun.
On the topic of cars, I go for economical diesels. Not very sporty, but you do clock up the miles driving from circuit to circuit.