As someone who had a c-reg Escort the best thing I can recommend is to get the shell completely stripped and then dipped/blasted.

if there is enough of a car left, then you will see what you have to work with (guarantee you not a lot left).

Honestly though, IMO I would use the good bits and find something else as you will be forever chasing rust round that shell. It will be in the seams, inside box sections and the roof will be shot, along with the chassis legs and a-pillars.

Been there, got the t-shirt.......