WHEW!!
If you choose to continue, here's what I'd do were it me:
1) pull the fender (its got to be repaired anyway),
2) pull the seat & carpet,
3) check inner panel for rust where it meets the floor,
4) check the floor for rust,
5) check the front suspension torsion bar perches for rust,
6) cut away the rocker about halfway back,
7) check the inner sill/castle rail for rust
8) check the bottom of the 'A' post for rust.
Then, get ready to totally disassemble the bodyshell for sandblasting....its the only way to find out how much of that stuff is surface and how much is deep other than probing with a screwdriver....or you could try grinding to find how far the rust goes....its labor either way: disassembly for sandblasting or grinding.
Then, worst case, get ready to toss the bodyshell!