That rust looks nasty. I can just tell you, rust repair is time consuming, doing it right and fitting sheet metal and so on takes a long time and if you think about ...idk 100$/h that adds up rather quickly. I have done rust repair myselfe and just replacing some floorboards on another car took at least 20 hours doing right... then say 5000 or so for a full paintjob and you are at 12k.
The areas in the pic, I would wirewheel all that and if there are no holes, put sealant in there, prime it and enjoy. Justtry to keep it from spreading.