Hmmmm.
Always a PITA this rust stuff!
The good news is, rust is far lighter than carbon fibre
I spent 10 years working on my '65 Datsun Fairlady repairing the major/obvious rust outs.
Part time of course!
[See the link in my signature.]
When it came time to engage a Body Shop to finish the job, the first thing they did was send the body and all panels to Minus Paint to strip off the surface rust, seam sealer and such.
Below and Hold! more rust!
BUT it is minor and reasonably easy to repair.
The moral of the story is, you won't know how bad it all is unless and until the panels are all stripped.
Then you are committed to spraying it with a primer to prevent it from deteriorating again.
Read, $$$ and time.
Unless you decide to develop the necessary skills and acquire the related tools, you will need the budget that you have been quoted.....and then some!
Having said all that, it is possible to learn welding, metal shaping and such and DIY.
It boils down to whether you want to see it all through or look for a better car.
Remember, they are all over 40 Y.O. now!