Firstly have a good read of the forum, thousands of posts I am sure they will answer 99% of you questions about the 126.
I would get a mechanic to have a look at the car to see what is needed for the mot. Rust will be the biggest problem, anything else is easy to fix. Welding can cost a lot (unless you want to learn to weld).
Pre 1973 cars are tax exempt I think. So you will need car tax.
As for classic car insurers, look in the back of classic car mags there are hundreds.
Last but not least welcome, and good luck with the car. Pop a few pictures on the forum, I am sure you will get comments back.