1. beginner level is, as labelled, for the beginners.. .
Once you know Tennis Elbow's controls, it's almost impossible to lose against it.
Club level is still quite easy, and once you know how to do accelerations, it shouldn't be a challenge anymore.
Junior level is the 1st really challenging level, and it's a medium difficulty. With training, most people should be able to beat it.
Pro level is "higher medium" difficulty, and can be very difficult for people not used to play video games.
Master level is quite hard, and Incredible level is insanely hard..!
So if you start a World Tour, I'd advise you to choose Junior or Pro level.
2. The choice of the character is purely aesthetic, and doesn't have any influence on the gameplay.
Only the characteristics influence it.