You should gain experience by joining our races.
I would jump straight to 125 if you want to buy something. If you buy an 80 or 100cc, there’s not a lot you can do with them other than practice, you may as well rent.
CK organise 125 races from time to time and they are great for getting some race experience, which as I found is totally different from going around and around on your own.
Coincidentally, I’m selling my 125MAX if you are interested, RM6000
Good luck and best wishes,
Chris