Welcome, Ditto what Kev has said.
Try a few of the online comparison sites for the best quotes or pop into a local insurance broker. I don't think you will find any company offering a full classic car policy for anyone under 21 though, but it will be a lot less than a Corsa, Twingo, KA ........... (sorry apologies for my bad language).
Re a 126, Bis or Aircooled? I am totally biased, I have never had a Bis so know very little about them. They are slightly faster, have a hatch back. There are less straight bolt on tuning parts for the Bis.
The aircooled is just great

Remember driving any classic is nothing like driving any modern car, Brakes are rubbish, they are slow, noisy and break down all the time. Weirdly though that is why I like them

For any questions have a really good read through old posts, the old bits on the forum will answer virtually any question you may have.
Most important, DO Not buy a 126 without knowing what you are looking for, or you will end up with a project that will just never get on the road.