buy a 125, there is plenty enough power to start with, they can reach around 70-75, or so depending on the bike, also if you haven't rode with gears before, then you may find it tricky to get used to, which you could do it at your own pace on your own bike, yes doing the lessons alone may come out more expensive. if you buy a 125, its a one off big bill, then you ride it on the roads, which is constant experience, you can practice mod one on a car park, which is more practice, take it on a lesson with you, which will be cheaper than renting theres!. the main thing is, buy something like a little ybr, they hold their value, so you don't lose much, its cheap insurance as it isn't some little rs125 getting rode around redlining everywhere. they are easy to ride, easy to learn on and enough power to get you through the test. ps even though you can't restrict a 600 down to a 125 power, if you could, with the weight of the bike you would ride around at 40 everywhere, which isnt fun at all. thought i would just throw that in there