CBT isn't a qualiication/licence, it's a 1 day trainning course, at the end of it you either get a bit of paper that says you're competent enough to go out and practice on your own or asked to come back for more training (IIRC it's free untill you either 'pass' or they tell you you're a no hope).
Step throughs are good for keeping legs warm in winter and people who can't lift their leg over a bike, that's about it. The handling is crap, there's very little frame holding the two ends together and usualy have single crown forks (on a bike weighing 100kg + rider!). They're Ok for pootling round town, but really not upto much, cornering's how I imagine driving one of those quarry trucks with articulated links between the cab and tipper for steering feels.
Something 125cc and fourstroke will be almost depreciation proof (think I 'lost' £100 in 18 months on an almost new CG125, the original owner only lost £300 off the RRP selling ti to me! And it'll still feel slow after you have your first go on a 'big' bike, which is probably what the school will give you to ride if you go back after your CBT for more training.
To put it in context, Caverndish puts out the same power as a moped through a 23mm tyre, and 50cc is smaller than a single espresso! That's the power output.