Hiya,
In all honesty it depends on the university. Obviously, Oxbridge uni's will need more A* than others. I have 7A's, 3B's, 2C's and a distinction*, and there's plenty I can still apply for. The main thing is getting the grades at A level. Strong A levels tend to compensate for poorer GCSE's. At the end of the day, you can only apply for 4 uni's for medicine, so as long as you don't get all D's, you should be fine:')
No, as I've previously said, A levels are worth a lot more than GCSE's. To a certain extent, GCSE's aren't as important as you might think.
Nope. They are used to give a basic idea of what you should get at AS, but A level predictions are made based on AS results. Annnd, it's not even that bad there - colleges can predict A's at A level even if you get a high C at AS, providing it's plausible.
Loads of people get in with out A* across the board.
Once you get to A level, you realise how easy GCSE's were. If you're thinking of applying to medicine, you must have a decent degree of intelligence to begin with, so just relax and I'm sure you'll do fine.
Good luck!