hello everyone! here's a few questions

1) I'm a D license holder (without P. I'm 26 already) and would like to take a B2 license for motorbikes. Do i have to take any theory and test again? How much would it be if i take my B2 now? is it a one day process?

2) I'm recently into naked bikes and found that there's a few 250cc bikes that is DAMN INTERESTING. Any recommendation? My budget is below 10k if possible. (plus minus around there)

3) I've shortlisted a few. Any comments on the bikes below/other recommendation? Here's a few i'll list down.
a) megelli 250sr
b) YSM T2
c) Demak DZM 200
d) Keeway RKV200g
e) Nimota CK9

4) Last question. I'm quite noob la in bikes so i was just wondering what determines the tyre width size? Big tyre bikes looks cool tho!
The reasons I will be getting a bike will be more like a weekend drive or "night drive a.k.a yumcha" only. No daily use intended.