125 is the maximum you can ride on a cbt. you don't need your theory at this point. You don't buy a 650 to do your das on, instead go to a school who will loan you a bike. the instructor will teach you how to pass mod 1 and mod 2 (for a fee) and take you for you tests. If you were going to do it on your own bike, you would need someone legal (and insured) to ride your bike to and from the test centre. or you could trailer it. You would then need to ensure you were fully insured to ride (expensive business pre-license on anything larger than a 125) and the bike was in a good state of repair. 2 stroke 125 engines sound like a raspberry, you're better looking for a YBR125 or CBF125. These are the basic japanese entry bikes and although they're not amazing sounding, nor awe inspiring in power, they are decent motorcycles. I shouldn't worry too much about how it sounds, or what you will look like while you're riding it, you're not going to be on it for long.