Depends how you want it done, metal, filler or body panel adhesive. Going through the same thing now. Found a guy that is doing all rust, paint, firewall, floor pan, bed repairs for 6-7K.
He is keeping the price low by blasting the rust, treating and sealing the remaining metal and then using panel adhesive. I undertand its what they use today to "weld" cars together at panel points, and is stronger than a weld until theres a fire at which point it melts.
I have a horrible rust issue at the cowl and firewall seam and was told that that the panel bond is 100% and the best way to go. Cannot comment as it isnt finished, but maybe you can use the body panel adhesive as a starting point to ck and balance aginst other types of repairs.