With regards the license:
The best option for you is to do the A category license which will allow you to ride anything up to 33bhp for 2 years, then if you are 21 before that, you can do another test to go beyond that, or wait until the 2 years are up and then ride anything you like
Training - you use a radio in which you can hear the instructor/examiner, but they can't hear you. All of the time on the training will be on a 125, my CBT I rode one of the school bikes but then did my test on a Honda Varadero (which I had bought myself). Much of the basics are taught (
Bike insurance wise- for me on a varadero on L plates it was 700 - but I was 17 and only had a provisional license. Last year, with a years no claims and being a year older, it was 327 something. This is TPFT though - but little point in getting comprehensive insurance unless you intend to crash (I didn't so therefore went TPFT)