You don't need a medical to start flight training, the medical is required at the time you go solo. In practice,the flight school will sort out a medical for you just before your first solo flight. Essentially if you can find your own way in to see the doc. then you are pretty much assured of a class 3 pass which isn't at all demanding. If you are planning to go commercial then it's a good idea to get a class 1 asap because that does require you to be in very fine form!