I believe it means that at 21, you could do the Category A Restricted I think - that means you can ride something restricted to 33 kw of power - most people I know, had an SV650 with a restriction kit fitted. There's a sticky at the top of this forum with all the updated details for licenses - I passed [thankfully] the year before they added all the extra layers to getting on two wheels.
If they are dealerships, then just do your best to gauge them really - I have brought cars in the past from dealerships, that have just turned out to be some backstreet yard - and the car has turned out to be ****. Fair enough, not all will be like that, and ones that look **** might be awesome.
As for 125 vs a 'big bike' - it's honestly a night and day difference! Fair enough, when I did DAS and jumped off a 125 to a Suzuki 500E, I was riding some cheap rotten Chinese 125 - so modern ones like the MT might be much better. But for me, a Big Bike just made the job of riding so much easier, and enjoyable.
I've said it before in posts, but a Big Bike will go better, stop better, turn better and feel better. It's not just about getting up to the speed limit, but how it gets there - I just found the 125 I rode was far too gutless, and influenced massively by the road contours and side winds. By comparison, the big Suzuki I jumped on, was just stable and planted. The difference was so extreme, that I personally could see how a 125 would put someone off of riding! The only thing a 125 might have over a big bike, is I expect they'll do 100+ mpg, versus 50+ from a big bike.
There's two (possibly more) schools of thought when it comes to bikes and their cc - some folk believe you ought to work your way up the ranks - 125/250/400/500/600/1000 and so on. Others, such as myself, believe you can ride whatever you like - provided you respect the bike and accept that you must be careful and have limits.
Riding is a process of constant learning regardless of where you start, you just need to do what you want, and what you believe is best for you