Only restriction for the provisional is 12bhp I think, it does not prevent you from using a geared bike. You can ride a 250 on a cbt when you're 18
no,
With a CBT:
at 16 you can have a 50cc limited to 31mph
at 17 you can ride a 125cc limited to 11hp (ish)
The full test is the same regardless whether you take it on a 125cc or 500cc bike apart from on a 500cc bike they'd expect you to hit higher speeds and accelerate quicker (i.e. your not allowed to creep arround at 125 speeds).
Cost is in the region of £600 with bike hire, training and all the tests (CBT, theory, 2 days training on big bikes, mod1 and mod2 tests).