Only you can decide whether it's worth saving, there's little doubt that you'll put more money into it than you will ever see back. (Unless you keep it decades.)

Regarding the £700 welding - is that for welding and brushing some back goo over when done or painting properly so that it looks as though the work has never been done? Clearly that is two different jobs. Once cutting away sills the job will only grow, it's never better inside than it looks outside...

If it were mine and I was going to save it I would strip the interior and boot trim leaving just the dash and drivers seat so still driveable. This will expose the top of the floor pan giving a good idea of what it's really like. Then take it round a few body shops and get an idea of costs as to putting it right and making it shiny all over. While the car is away having new metal and paint the interior can be worked on....

I hope you save it, it's a good looking motor.