1. A Levels and Grades mean nothing. I don't have many good grades at all but I've managed to start up 2 businesses and I'm only 19. I've also given my previous jobs business & marketing advice and they've made a lot of money from me. But, for one, I would recommend taking Media Studies - It will look good on your CV.
2. You need to think quickly and be able to solve any problem that could arise. You need to be aware of the current economic climate and current affairs in the world. You need to know what you are talking about.
3. I love anything to do with business. And marketing falls into it nicely, so yes, I definitely feel it's a good job. You can earn a lot of money from business & marketing, and I find marketing to be quite easy if I'm honest.
4. No. Forget uni. Many successful business owners dropped out of school at a young age and never went to uni. It's not all about grades, as I've said. I didn't go to uni, I didn't study business or marketing. I learnt it all myself from the age of 16.